Education · Tier 5

· Last Reviewed May 15, 2026· PSI Editorial Board· Independent

How Does PSI Grade Peptide Evidence?

The methodology reference for PSI's four-tier evidence framework anchored in FDA Drug Approval Process, ICH E6 Good Clinical Practice, GRADE methodology, and Editorial Standards.

PSI uses four evidence tiers from preclinical to FDA-approved.

Visitor-facing labels are FDA Approved, Human Trials, Animal Studies, and Preclinical.

Every compound on PSI carries a locked evidence level.

The level is traceable to peer-reviewed primary sources.

4 Tiers

Evidence Framework

L1 preclinical, L2 animal studies, L3 human trials, L4 FDA-approved with locked tier per compound

Locked

Per Compound

Every compound on PSI declares one evidence level traceable to peer-reviewed primary sources

Visitor

Plain-Language Labels

FDA Approved, Human Trials, Animal Studies, Preclinical replace L1-L4 internal notation in visitor view

GRADE

Methodology Anchor

Anchored in GRADE, USPSTF, Cochrane Handbook, AHRQ evidence-based practice framework

Quick Answer

PSI uses a four-tier evidence framework. The framework distinguishes preclinical, animal studies, human trials, and FDA-approved compounds. Every compound declares one tier traceable to peer-reviewed primary sources.

L1 preclinical covers in vitro studies, mechanistic biology, and computational modeling. The tier excludes animal and human data. Examples include cell culture binding assays and structure-activity modeling.

L2 animal studies covers rodent, large-animal, and other animal models. The tier excludes human trial data. Most compounded research peptides without human trial evidence sit at L2. Examples include compounded BPC-157 and many emerging compounds.

L3 human trials covers any human trial data. The tier includes pilot studies, Phase 1 first-in-human, Phase 2 dose-finding, and Phase 3 pivotal trials. Compounds with human evidence that have not received FDA pre-market approval sit at L3. Examples include compounded Thymosin Alpha-1 in international Zadaxin approval contexts.

L4 FDA-approved covers compounds with completed FDA pre-market review and approved indication-specific labels. The tier is the strongest evidence anchoring on PSI. Examples include Semaglutide (Wegovy NDA 215256), Tirzepatide (Zepbound NDA 217806), and Teriparatide (Forteo NDA 021318).

Visitor-facing labels are FDA Approved, Human Trials, Animal Studies, and Preclinical. The labels replace internal L1-L4 notation in compound pages, condition pages, comparison pages, and bloodwork pages. The framework supports physician archetype trust through transparent claim traceability.

The methodology is anchored in established evidence-hierarchy frameworks. The frameworks include GRADE, USPSTF Procedure Manual, Cochrane Handbook, AHRQ evidence-based practice, and the FDA Drug Approval Process. See Editorial Standards for the full methodology documentation. Also see Compounded vs FDA-Approved Peptides for the regulatory pathway distinction.

The four-tier framework anchors transparent communication. L1 preclinical covers in vitro and mechanistic studies. L2 animal studies cover rodent and large-animal work without human data. L3 human trials cover any human trial data including pilot, Phase 1, Phase 2, and Phase 3 studies. L4 FDA-approved covers compounds with completed FDA pre-market review and approved indication-specific labels. The framework is anchored in established evidence-hierarchy methodology including GRADE, USPSTF procedure manual, Cochrane Handbook, and AHRQ evidence-based practice framework. Every compound page on PSI declares its evidence level with primary-source citations.

PSI EVIDENCE FRAMEWORK

At a Glance: Evidence Levels Explained

Tier	Subtitle	Animal Evidence	Human Evidence	Definition and Examples
L1 Preclinical tier	In vitro and mechanistic studies without animal or human data	—	Limited	Cell culture, binding assays, computational modeling, structure-activity studies. Foundational but limited predictive validity for clinical translation
L2 Animal Studies tier	Rodent and other animal models without human trial data	—	Limited	Mouse, rat, rabbit, dog, pig, and primate models. Most compounded research peptides without human evidence sit at L2
L3 Human Trials tier	Any human trial data including pilot, Phase 1, Phase 2, Phase 3	—	Moderate	The tier captures full breadth of human evidence including trials that did not lead to FDA approval. Quality varies substantially within the tier
L4 FDA-Approved tier	Compounds with completed FDA pre-market review and approved labels	—	Strong	Wegovy, Ozempic, Zepbound, Mounjaro, Forteo, Tymlos, Evenity, Tesamorelin, Vyleesi, somatropin, and other FDA-approved peptide drugs
Cross-tier disambiguation	Decision rules for compounds spanning multiple tiers	—	Moderate	Compound's evidence level is highest tier with substantive evidence. Thymosin Alpha-1 sits at L3 internationally, L2 in many US off-label indications
Visitor-facing labels	FDA Approved, Human Trials, Animal Studies, Preclinical	—	Strong	Replace L1-L4 internal notation in visitor view. Plain-language labels support physician archetype trust through readable claim tier
Per-compound lock	Single declared evidence level per compound on PSI	—	Strong	Locked in peptides.ts data layer with primary-source citations. Single source of truth across compound, condition, comparison, and bloodwork pages
Methodology anchoring	GRADE, USPSTF, Cochrane, AHRQ, FDA Drug Approval Process	—	Strong	PSI methodology is documented in Editorial Standards and anchored in established evidence-hierarchy frameworks for clinical research

Six Things You Need to Know About PSI Evidence Levels

This page covers PSI's four-tier evidence framework methodology. Section one covers the four tiers from preclinical to FDA-approved with examples per tier. Section two covers cross-tier disambiguation rules for compounds spanning tiers. Section three covers visitor-facing labels and the single-source-of-truth principle. Section four covers methodology anchoring in GRADE, USPSTF, Cochrane, AHRQ, and FDA frameworks.

PSI Uses a Four-Tier Evidence Framework From Preclinical to FDA-Approved

PSI's evidence-level framework distinguishes four tiers. L1 preclinical covers in vitro and mechanistic studies. L2 animal studies covers rodent and other animal models without human data. L3 human trials covers any human trial data. L4 FDA-approved covers compounds with completed FDA pre-market review and approved indication-specific labels.

The four-tier framework provides a transparent classification system for peptide therapy evidence quality. The framework supports physician archetype trust by making evidence quality explicit per compound. Every compound page on PSI declares one locked evidence level. The level is traceable to peer-reviewed primary sources. The framework anchors compound pages, condition pages, comparison pages, bloodwork pages, and the FDA Status Tracker. The transparency principle is that every claim is traceable to evidence tier. PSI uses visitor-facing labels FDA Approved, Human Trials, Animal Studies, and Preclinical rather than the internal L1-L4 notation. The labels are designed for plain-language reading by physicians, researchers, and patients. The Editorial Standards page at peptidescienceinstitute.org/editorial-standards documents the full methodology including tier definitions, cross-tier disambiguation rules, source-quality hierarchy, and primary-source citation requirements.

L1 Preclinical Is the Foundational Tier But Has Limited Predictive Validity

L1 preclinical covers in vitro studies, mechanistic biology, and computational modeling without animal or human data. The tier is foundational for understanding mechanism but has limited predictive validity for clinical translation. Most novel research peptides start at L1 before advancing to animal studies.

L1 preclinical evidence includes cell culture studies, receptor binding assays, enzyme kinetics studies, computational modeling of protein-protein interactions, structure-activity relationship studies, and other in vitro work. The tier captures the mechanistic foundation of any peptide therapy. Examples include early structure-activity work on GLP-1 receptor binding, cell culture studies of BPC-157 cytoprotection, and mechanistic studies of thymosin signaling pathways. The tier has limited predictive validity for clinical outcomes for several reasons. Cell culture conditions differ substantially from in vivo physiology. Receptor binding does not automatically predict therapeutic effect at organism level. Computational modeling captures structural features but not pharmacokinetics or off-target effects. The tier is necessary but not sufficient for clinical translation. Most compounds with only L1 evidence carry conservative claims framing on PSI. The framework emphasizes that L1 evidence is foundational rather than determinative for therapeutic decisions.

L2 Animal Studies Are Mechanistically Informative But Not Clinically Predictive

L2 animal studies covers rodent, large-animal, and other animal model studies without human trial data. The tier provides in vivo evidence for mechanism, safety pharmacology, and dose-ranging but does not automatically predict clinical outcomes in humans.

L2 animal studies include mouse, rat, rabbit, dog, pig, and non-human primate research. The tier captures in vivo evidence for biological activity, pharmacokinetics, dose-ranging, and acute and subchronic safety. Animal studies are essential for FDA Investigational New Drug (IND) submissions per the FDA Drug Approval Process. The studies inform Phase 1 first-in-human dose selection. Most compounded research peptides without human trial evidence sit at L2. Examples include compounded BPC-157 (primarily rodent tendon healing and gastrointestinal models), compounded TB-500 thymosin beta-4 fragment (rodent tissue repair models), epitalon (rodent longevity studies), and many emerging research peptides. The tier has substantial limitations for clinical prediction. Animal physiology differs from human physiology in many tissue-specific ways. Animal-to-human dose scaling is imperfect. Effective doses in animal models may not translate to safe doses in humans. Many compounds with strong animal evidence have failed Phase 1 or Phase 2 human trials due to off-target effects, pharmacokinetic differences, or species-specific receptor pharmacology. The tier captures essential preclinical work but should not be confused with clinical efficacy evidence.

L3 Human Trials Captures the Full Breadth of Human Evidence

L3 human trials covers any human trial data including pilot studies, Phase 1 first-in-human safety studies, Phase 2 dose-finding and proof-of-concept studies, and Phase 3 pivotal efficacy and safety trials. The tier captures the full breadth of human evidence including trials that did not lead to FDA approval.

L3 human trials tier is intentionally broad to capture compounds with human evidence that have not received FDA pre-market approval. The tier includes academic Phase 1 first-in-human safety studies, single-site pilot trials, multi-site Phase 2 dose-finding studies, Phase 3 pivotal trials that did not result in FDA approval, and registry studies with prospective human outcome data. The tier excludes purely retrospective observational studies, case reports, and case series. Quality within the L3 tier varies substantially. A 10-patient open-label pilot study and a 500-patient randomized controlled trial both fall within L3 but provide very different evidence quality. The Editorial Standards methodology distinguishes within-tier quality through study-design annotations on compound pages. Examples of L3 compounds on PSI include compounded thymosin alpha-1 (international Zadaxin approval in approximately 35 countries with substantial hepatitis B and C clinical evidence but not FDA-approved in US), compounded cerebrolysin (substantial international clinical evidence including Phase III stroke and dementia trials but not FDA-approved), and some emerging research peptides with Phase 1 or Phase 2 human evidence.

L4 FDA-Approved Is the Definitive Top Tier

L4 FDA-approved covers compounds with completed FDA pre-market review and approved indication-specific labels. The tier is the strongest evidence anchoring on PSI. Examples include Wegovy semaglutide, Zepbound tirzepatide, Forteo teriparatide, and other FDA-approved peptide drugs.

L4 FDA-approved tier captures the rigorous FDA pre-market review framework. The framework includes preclinical safety pharmacology, Phase 1 first-in-human safety and pharmacokinetics, Phase 2 dose-finding and proof-of-concept, and Phase 3 pivotal efficacy and safety trials at therapeutic dose. The framework operates under the FDA New Drug Application (NDA) or Biologics License Application (BLA) pathway. Post-marketing commitments may include Phase 4 trials, registry studies, and FDA MedWatch adverse event surveillance. Examples of L4 peptide drugs on PSI include Wegovy semaglutide for chronic weight management (NDA 215256, anchored in STEP trial program and SELECT 2023 cardiovascular outcomes), Ozempic semaglutide for type 2 diabetes (NDA 209637, anchored in SUSTAIN trial program), Zepbound and Mounjaro tirzepatide (NDA 217806 and NDA 215866, anchored in SURMOUNT and SURPASS trial programs), Forteo teriparatide (NDA 021318, anchored in VERT trial), Tymlos abaloparatide (NDA 208743, anchored in ACTIVE trial), Evenity romosozumab (NDA 761062 with cardiovascular boxed warning, anchored in FRAME and ARCH trials), Tesamorelin Egrifta (NDA 022505 for HIV-associated lipodystrophy), and Vyleesi bremelanotide (NDA 210557 for HSDD in premenopausal women). FDA approval is indication-specific. A compound may be FDA-approved for one indication but operate at lower evidence tier for off-label indications.

Cross-Tier Disambiguation Rules Apply to Compounds Spanning Multiple Tiers

PSI uses cross-tier disambiguation rules for compounds with evidence spanning multiple tiers. A compound's declared evidence level is the highest tier with substantive evidence. The rule supports honest claim communication while avoiding both overstatement and understatement.

Cross-tier disambiguation matters because many compounds have evidence at multiple tiers simultaneously. Examples include compounds with extensive preclinical mechanistic work plus animal studies plus limited Phase 1 human data. The PSI rule sets the declared evidence level to the highest tier with substantive evidence. Substantive evidence at human trials tier requires more than a single small case series. Substantive evidence at FDA-approved tier requires actual FDA approval, not pending review or shortage-list status. Compounds with FDA approval in one indication but emerging human data in others have nuanced presentation. Wegovy semaglutide is FDA-approved (L4) for chronic weight management but the off-label use for non-obese contexts operates as off-label prescribing per AMA Code 1.1.5 framework rather than L4 evidence for that specific use. Thymosin alpha-1 is approved internationally as Zadaxin (L3 with substantial hepatitis B and C clinical evidence in 35-country approval contexts) but operates as compounded preparation in the US with off-label indications spanning L1 to L3 depending on the specific indication. The Editorial Standards page documents the disambiguation rules and provides examples per common edge case.

The L4 FDA-Approved framework: pre-market review and Phase 3 evidence

The definitive top tier on PSI anchored in FDA New Drug Application pathway

L4 FDA-approved tier captures compounds with completed FDA pre-market review under the New Drug Application (NDA) or Biologics License Application (BLA) framework. The pre-market review includes preclinical safety pharmacology per FDA guidance, Phase 1 first-in-human safety and pharmacokinetics typically in healthy volunteers, Phase 2 dose-finding and proof-of-concept in target patient population, and Phase 3 pivotal efficacy and safety trials at proposed therapeutic dose in larger patient cohorts. The framework operates under 21 CFR 312 (IND framework) and 21 CFR 314 (NDA framework).

Post-marketing commitments may include Phase 4 trials, registry studies, and FDA MedWatch adverse event surveillance. Some FDA-approved peptides carry boxed warnings or REMS programs based on Phase 3 safety findings. Examples include Evenity romosozumab with cardiovascular boxed warning per April 2019 FDA action and Vyleesi bremelanotide with REMS for HSDD indication. The post-marketing framework provides ongoing safety signal monitoring across the FDA-approved peptide drug class.

Examples of L4 peptide drugs on PSI include the GLP-1 receptor agonist class for chronic weight management and type 2 diabetes, the anabolic osteoporosis peptide class for high fracture risk indications, Tesamorelin Egrifta for HIV-associated lipodystrophy, and Vyleesi bremelanotide for HSDD in premenopausal women. Cardiovascular outcomes evidence anchors many of the metabolic class including SELECT 2023, SUSTAIN-6, LEADER, and SURMOUNT-1 trial programs. The L4 tier provides the strongest evidence anchoring on PSI.

The L3 Human Trials tier: pilot, Phase 1, Phase 2, Phase 3 evidence

Any human trial data including compounds that have not received FDA pre-market approval

L3 human trials tier covers any human trial data. The tier includes academic Phase 1 first-in-human safety studies, single-site pilot trials, multi-site Phase 2 dose-finding studies, Phase 3 pivotal trials that did not result in FDA approval, and registry studies with prospective human outcome data. The tier excludes purely retrospective observational studies, case reports, and case series.

Quality within the L3 tier varies substantially. A 10-patient open-label pilot study and a 500-patient randomized controlled trial both fall within L3 but provide very different evidence quality. The Editorial Standards methodology distinguishes within-tier quality through study-design annotations on compound pages. Trial type, sample size, randomization, blinding, control comparator, and outcome definitions all inform within-tier assessment.

Examples of L3 compounds on PSI include compounded thymosin alpha-1 (international Zadaxin approval in approximately 35 countries with substantial hepatitis B and C clinical evidence but not FDA-approved in US), compounded cerebrolysin (substantial international clinical evidence including Phase III stroke and dementia trials but not FDA-approved), retatrutide (Phase 3 TRIUMPH trial program ongoing for weight management and type 2 diabetes per Eli Lilly development pipeline), and other compounds with Phase 1 through Phase 3 human evidence. Compounds in this tier carry honest framing that the evidence base is human but the FDA pre-market review has not been completed for the specific indication.

The L2 Animal Studies tier: rodent and large-animal model evidence

In vivo preclinical evidence without human trial data

L2 animal studies tier covers rodent, large-animal, and other animal model studies without human trial data. The tier includes mouse, rat, rabbit, dog, pig, and non-human primate research. The tier captures in vivo evidence for biological activity, pharmacokinetics, dose-ranging, and acute and subchronic safety. The studies are essential for FDA Investigational New Drug (IND) submissions per 21 CFR 312 framework.

The tier has substantial limitations for clinical prediction. Animal physiology differs from human physiology in many tissue-specific ways including receptor pharmacology, metabolic enzyme expression, immune cell repertoire, microbiome composition, and tissue regenerative capacity. Animal-to-human dose scaling uses body surface area, allometric scaling, or pharmacokinetic modeling but remains imperfect. Effective doses in animal models may not translate to safe or effective doses in humans. Many compounds with strong animal evidence have failed Phase 1 or Phase 2 human trials due to off-target effects, pharmacokinetic differences, or species-specific receptor pharmacology.

Most compounded research peptides without human trial evidence sit at L2 on PSI. Examples include compounded BPC-157 with primarily rodent tendon healing and gastrointestinal cytoprotection model evidence, compounded TB-500 thymosin beta-4 fragment with rodent tissue repair model evidence, epitalon with rodent longevity model evidence (Khavinson laboratory program), MOTS-c with rodent metabolic and exercise mimicry model evidence, humanin with rodent neuroprotection model evidence, and many emerging research peptides. Compounds in this tier carry honest framing that the evidence base is animal model rather than human clinical.

The L1 Preclinical tier: in vitro and mechanistic foundation

Cell culture, binding assays, computational modeling without animal or human data

L1 preclinical tier covers in vitro studies, mechanistic biology, and computational modeling without animal or human data. The tier includes cell culture studies, receptor binding assays, enzyme kinetics studies, computational modeling of protein-protein interactions, structure-activity relationship studies, and other in vitro work. The tier captures the mechanistic foundation of any peptide therapy.

The tier has limited predictive validity for clinical outcomes. Cell culture conditions differ substantially from in vivo physiology including absent vascularization, simplified extracellular matrix, absent immune system interactions, and absent metabolic clearance pathways. Receptor binding does not automatically predict therapeutic effect at organism level. Computational modeling captures structural features but not pharmacokinetics, off-target effects, or species-specific pharmacology. The tier is necessary but not sufficient for clinical translation.

Examples of L1 preclinical work spanning the peptide class include early structure-activity work on GLP-1 receptor binding and selectivity, cell culture studies of BPC-157 cytoprotection mechanisms, mechanistic studies of thymosin signaling pathways, in vitro studies of mitochondrial peptide function (MOTS-c, humanin, SS-31), and computational modeling of novel peptide therapeutics. Most compounds with only L1 evidence carry conservative claims framing on PSI. The framework emphasizes that L1 evidence is foundational rather than determinative for therapeutic decisions. Compounds in this tier should not be presented with strong clinical claims regardless of mechanistic plausibility.

Research Suggests

Direction

PSI uses a four-tier evidence framework distinguishing preclinical, animal studies, human trials, and FDA-approved compounds. The framework supports transparent claim communication and physician archetype trust.

The four-tier framework (L1 preclinical, L2 animal studies, L3 human trials, L4 FDA-approved) provides a transparent classification system for peptide therapy evidence quality. Every compound page on PSI declares one locked evidence level traceable to peer-reviewed primary sources. The framework anchors compound pages, condition pages, comparison pages, bloodwork pages, and the FDA Status Tracker. Visitor-facing labels (FDA Approved, Human Trials, Animal Studies, Preclinical) replace internal L1-L4 notation. The methodology is anchored in established evidence-hierarchy frameworks including GRADE (Grading of Recommendations Assessment, Development, and Evaluation), USPSTF Procedure Manual, Cochrane Handbook for Systematic Reviews of Interventions, AHRQ Evidence-Based Practice Center framework, and the FDA Drug Approval Process under 21 CFR 312 (IND) and 21 CFR 314 (NDA).

Strongest evidence

L4 FDA-approved tier provides the strongest evidence anchoring on PSI. The tier captures compounds with completed FDA pre-market review and approved indication-specific labels.

L4 FDA-approved peptide drugs anchored in Phase 3 trial evidence provide the strongest framework on PSI. The FDA pre-market review framework includes preclinical safety pharmacology, Phase 1 first-in-human safety and pharmacokinetics, Phase 2 dose-finding and proof-of-concept, and Phase 3 pivotal efficacy and safety trials. The framework operates under 21 CFR 312 (IND framework) and 21 CFR 314 (NDA framework) with ICH E6 Good Clinical Practice guidance. Examples of L4 peptide drugs include the GLP-1 receptor agonist class (Wegovy NDA 215256, Ozempic NDA 209637, Zepbound NDA 217806, Mounjaro NDA 215866, Saxenda, Victoza), the anabolic osteoporosis peptide class (Forteo NDA 021318, Tymlos NDA 208743, Evenity NDA 761062), Tesamorelin (Egrifta NDA 022505), and Vyleesi (NDA 210557). Cardiovascular outcomes evidence anchors many of the metabolic class including SELECT 2023 NEJM, SUSTAIN-6 NEJM 2016, LEADER NEJM 2016, and SURMOUNT-1 NEJM 2022.

Limitations

Evidence-tier framework is necessarily a simplification of complex evidence-quality assessment. Within-tier variation is substantial and the framework does not capture every dimension of evidence quality.

The four-tier framework is a useful simplification but does not capture all dimensions of evidence quality. Within-tier variation is substantial. An L3 compound with a single 10-patient pilot study and an L3 compound with multiple Phase 3 trials both carry the L3 designation but provide very different evidence quality. The Editorial Standards methodology supplements the tier designation with study-design annotations including trial type, sample size, randomization, blinding, comparator, and outcome definitions. The framework also does not capture indication-specific nuance. A compound may be FDA-approved (L4) for one indication but operate at lower evidence tier for off-label indications. The framework treats the highest-tier indication as the compound's declared tier but indication-specific pages note off-label tier separately. The framework does not assess risk-benefit ratio, regulatory pathway nuances beyond US FDA framework (international approvals like Zadaxin thymosin alpha-1 in 35 countries), or post-marketing safety signals beyond what is captured in FDA labels. Specialty coordination supports comprehensive evidence interpretation beyond tier designation alone.

Assessment

The framework establishes transparent evidence communication anchored in established methodology. The framework is necessary infrastructure for credible peptide research communication.

PSI's reading: the four-tier evidence framework provides transparent and verifiable evidence communication infrastructure. The framework is anchored in established evidence-hierarchy methodology including GRADE, USPSTF, Cochrane Handbook, AHRQ evidence-based practice, FDA Drug Approval Process, and ICH E6 Good Clinical Practice. Every compound on PSI carries one locked evidence level traceable to peer-reviewed primary sources documented in the data layer. Visitor-facing labels (FDA Approved, Human Trials, Animal Studies, Preclinical) replace internal L1-L4 notation for plain-language readability. The framework supports physician archetype trust through readable claim tier and primary-source citation traceability. The transparency principle is that every claim is traceable to evidence tier. Cross-tier disambiguation rules apply for compounds spanning multiple tiers with the declared level set to the highest tier with substantive evidence. The Editorial Standards page documents the full methodology including tier definitions, disambiguation rules, source-quality hierarchy, and primary-source citation requirements. The framework is permanent infrastructure protecting PSI's communication credibility across compound pages, condition pages, comparison pages, bloodwork pages, and the FDA Status Tracker.

How to Approach Your Decision

Limitations and Caveats

Within-tier variation is substantial. An L3 compound with a single pilot study and an L3 compound with multiple Phase 3 trials both carry the L3 designation but provide different evidence quality.
The framework does not capture indication-specific nuance fully. A compound may be FDA-approved for one indication but operate at lower evidence tier for off-label indications.
International regulatory approvals beyond US FDA are noted but do not change the L4 designation. Zadaxin thymosin alpha-1 has 35-country approval but is not FDA-approved in the US.
Risk-benefit ratio is separate from evidence tier. A high-tier compound may still carry meaningful safety considerations including boxed warnings or REMS programs.
Post-marketing safety signals require ongoing monitoring beyond initial FDA approval. FDA MedWatch and post-marketing studies inform safety understanding.
The framework does not substitute for individualized clinical judgment. Specialty coordination supports comprehensive evidence interpretation beyond tier designation alone.
Evidence-tier designations may evolve with new research. New Phase 3 trials may shift a compound from L2 to L3, or new FDA approvals may shift L3 to L4.
Some compounds have evidence that does not fit cleanly into tier framework. Emerging biomarker-based or registry-based evidence may not map to traditional trial-tier framework.

What's Marketed vs What's Studied

7 common claims, corrected.

“All peptide therapy is FDA-approved.”

Only some peptides are FDA-approved. Examples of FDA-approved peptides (L4 tier) include Wegovy, Ozempic, Zepbound, Mounjaro, Forteo, Tymlos, Evenity, Tesamorelin Egrifta, and Vyleesi. Many compounds operate at L1 preclinical, L2 animal studies, or L3 human trials tier rather than L4 FDA-approved.

“Animal study evidence proves clinical efficacy in humans.”

L2 animal studies provide in vivo preclinical evidence but do not automatically predict clinical outcomes in humans. Animal physiology differs from human physiology in many tissue-specific ways. Many compounds with strong animal evidence have failed Phase 1 or Phase 2 human trials.

“Mechanistic plausibility from preclinical work substitutes for clinical evidence.”

L1 preclinical evidence including in vitro studies and mechanistic modeling is foundational but has limited predictive validity for clinical outcomes. Mechanistic plausibility is necessary but not sufficient for clinical translation. Compounds with only L1 evidence carry conservative claims framing.

“FDA approval for one indication means approval for all indications.”

FDA approval is indication-specific. A compound may be FDA-approved (L4) for one indication but operate at lower evidence tier for off-label indications. Off-label prescribing operates under AMA Code of Medical Ethics 1.1.5 framework with documented risk-benefit assessment.

“International regulatory approval is equivalent to FDA approval.”

International approvals like Zadaxin thymosin alpha-1 in approximately 35 countries are noted but do not change US FDA tier designation. The framework treats L4 as FDA-approved specifically. International approval is reflected in the L3 tier for compounds with international clinical evidence.

“Higher evidence tier means safer compound.”

Evidence tier captures evidence quality not risk-benefit ratio. A high-tier compound may carry meaningful safety considerations including boxed warnings (Evenity romosozumab cardiovascular boxed warning) or REMS programs (Vyleesi bremelanotide HSDD REMS). Evidence quality and safety profile are distinct dimensions.

“Pilot studies and Phase 3 trials provide equivalent evidence within L3 tier.”

Within-tier variation is substantial. A 10-patient open-label pilot study and a 500-patient randomized controlled trial both fall within L3 but provide different evidence quality. Editorial Standards distinguishes within-tier quality through study-design annotations including trial type, sample size, randomization, blinding, and comparator.

Common Questions

What is PSI's evidence-level framework?

PSI uses a four-tier framework distinguishing preclinical, animal studies, human trials, and FDA-approved compounds. L1 preclinical covers in vitro and mechanistic studies. L2 animal studies covers rodent and other animal models without human data. L3 human trials covers any human trial data. L4 FDA-approved covers compounds with completed FDA pre-market review.

What are the visitor-facing evidence labels?

Visitor-facing labels are FDA Approved, Human Trials, Animal Studies, and Preclinical. The labels replace internal L1-L4 notation in compound pages, condition pages, comparison pages, and bloodwork pages. The plain-language labels support physician archetype trust and patient readability.

How does PSI determine a compound's evidence level?

PSI declares each compound's evidence level based on the highest tier with substantive evidence. The Editorial Standards methodology documents the disambiguation rules. Primary-source citations support the declared level. The level is locked in the peptides.ts data layer as single source of truth across compound, condition, comparison, and bloodwork surfaces.

What is the difference between L1 preclinical and L2 animal studies?

L1 preclinical covers in vitro studies, mechanistic biology, and computational modeling without animal or human data. L2 animal studies covers rodent and other animal model studies with in vivo evidence but no human trial data. The L2 tier provides organism-level evidence while L1 captures the mechanistic foundation.

What is the difference between L3 human trials and L4 FDA-approved?

L3 human trials covers any human trial data including pilot, Phase 1, Phase 2, and Phase 3 studies. L4 FDA-approved covers compounds with completed FDA pre-market review and approved indication-specific labels. A compound can have substantial human trial evidence (L3) without FDA approval if pre-market review was not completed for the specific indication.

How does PSI handle cross-tier compounds spanning multiple tiers?

PSI uses cross-tier disambiguation rules. A compound's declared evidence level is the highest tier with substantive evidence. Substantive evidence at human trials tier requires more than a single small case series. Substantive evidence at FDA-approved tier requires actual FDA approval. The Editorial Standards page documents the rules with examples.

Why use L1 to L4 internally but FDA Approved / Human Trials / Animal Studies / Preclinical for visitors?

Internal L1-L4 notation supports concise data-layer representation and developer tooling. Visitor-facing labels (FDA Approved, Human Trials, Animal Studies, Preclinical) support plain-language readability for physicians, researchers, and patients. The dual notation maintains transparency without sacrificing readability.

What is GRADE methodology?

GRADE (Grading of Recommendations Assessment, Development, and Evaluation) is an established evidence-hierarchy framework used by clinical practice guideline developers. GRADE assesses evidence quality across study design, risk of bias, inconsistency, indirectness, imprecision, and publication bias dimensions. PSI's four-tier framework is informed by GRADE principles while simplified for peptide-specific application.

How does PSI's framework compare to USPSTF or Cochrane?

USPSTF (US Preventive Services Task Force) uses A, B, C, D, and I grade designations for clinical preventive services recommendations. Cochrane Handbook for Systematic Reviews documents systematic review methodology with evidence quality assessment. PSI's four-tier framework is informed by both frameworks but simplified for peptide-specific application.

How does PSI handle compounds with FDA approval in one indication but emerging human data in others?

Compounds with FDA approval in one indication carry the L4 designation for that indication. For off-label or emerging indications, the compound operates as off-label prescribing per AMA Code of Medical Ethics 1.1.5 framework rather than L4 evidence for that specific use. The indication-specific pages note off-label tier separately.

What is the FDA Drug Approval Process?

The FDA Drug Approval Process covers the New Drug Application (NDA) framework under 21 CFR 314 and the Biologics License Application (BLA) framework under 21 CFR 601. The process includes preclinical safety pharmacology, Phase 1 first-in-human safety, Phase 2 dose-finding, and Phase 3 pivotal efficacy and safety trials. Post-marketing commitments include Phase 4 trials and FDA MedWatch surveillance.

What is ICH E6 Good Clinical Practice?

ICH E6 Good Clinical Practice is the International Council for Harmonisation guideline for the design, conduct, monitoring, recording, analysis, and reporting of clinical trials. The guideline provides a unified standard for the European Union, Japan, United States, Canada, Switzerland, and other ICH regions for trial conduct. FDA-approved peptide drugs route through ICH E6-compliant trials.

What is 21 CFR 312?

21 CFR 312 (Code of Federal Regulations Title 21 Part 312) is the FDA Investigational New Drug (IND) framework regulation. The framework governs the use of investigational drugs in human research. The framework requires preclinical safety pharmacology, animal toxicology, and Phase 1 first-in-human safety study protocols before authorizing human investigation of a new drug.

What is 21 CFR 314?

21 CFR 314 (Code of Federal Regulations Title 21 Part 314) is the FDA New Drug Application (NDA) framework regulation. The framework governs the FDA pre-market review process for new drug applications including the chemistry, manufacturing, and controls section, the clinical pharmacology section, the clinical efficacy and safety section, and the labeling section.

How does PSI's framework handle compounds with primarily international evidence?

Compounds with primarily international evidence are classified per the highest applicable tier. Thymosin alpha-1 has substantial international clinical evidence in 35-country Zadaxin approval contexts and is classified L3 human trials. The compound is not FDA-approved in the US so does not qualify for L4. International approval status is reflected in compound page metadata.

Can a compound's evidence level change over time?

Yes. Evidence-tier designations may evolve with new research. New Phase 3 trials may shift a compound from L2 animal studies to L3 human trials. New FDA approvals may shift L3 human trials to L4 FDA-approved. PSI's Editorial Standards review cadence updates compound-level evidence designations as new evidence becomes available.

Where is the full methodology documented?

The Editorial Standards page at peptidescienceinstitute.org/editorial-standards documents the full methodology including tier definitions, cross-tier disambiguation rules, source-quality hierarchy, primary-source citation requirements, and review cadence. The page is the canonical reference for PSI's evidence-tier framework methodology.

How does evidence tier relate to AMA Code 1.1.5 framework?

AMA Code of Medical Ethics 1.1.5 framework governs off-label and compounded prescribing decisions. The framework requires documented risk-benefit assessment, FDA-approved alternatives considered, monitoring requirements, and patient understanding. Evidence tier informs the risk-benefit assessment but does not substitute for the AMA Code 1.1.5 documentation framework. Both frameworks operate together to support transparent prescribing decisions.

Sourcing Checklist

Look for the declared evidence level on every compound page.
PSI compound pages display the locked evidence level using visitor-facing labels (FDA Approved, Human Trials, Animal Studies, Preclinical).
Verify primary-source citations support the declared level.
Every claim on PSI links to peer-reviewed primary sources documented in the data layer. The transparency principle is that every claim is traceable to evidence tier.
Note within-tier variation through study-design annotations.
L3 human trials includes pilot studies and Phase 3 trials with substantially different evidence quality. Editorial Standards annotates trial type, sample size, randomization, and comparator.
Distinguish FDA-approved indication from off-label use.
A compound's L4 designation applies to FDA-approved indications. Off-label use operates under AMA Code of Medical Ethics 1.1.5 framework rather than L4 evidence for that specific use.
Note international regulatory approvals where applicable.
Compounds with international approval (e.g., Zadaxin thymosin alpha-1 in 35 countries) carry L3 human trials designation rather than L4 FDA-approved which requires US FDA approval specifically.
Separate evidence quality from safety profile.
Evidence tier captures evidence quality. Safety profile is a separate dimension including boxed warnings, REMS programs, and adverse event surveillance findings.
Review the Editorial Standards methodology documentation.
Editorial Standards page documents the full framework including tier definitions, disambiguation rules, source-quality hierarchy, and review cadence.
Expect specialty coordination for comprehensive evidence interpretation.
Evidence-tier framework supports communication but does not substitute for individualized clinical judgment. Specialty coordination supports comprehensive evidence interpretation beyond tier designation alone.
Discuss evidence tier with prescribing physician for any peptide therapy decision.
AMA Code 1.1.5 framework requires documented risk-benefit assessment incorporating evidence quality. Evidence tier informs the discussion but the physician integrates tier with patient-specific clinical context.

Regulatory Context

Evidence-tier designations evolve as new research emerges. New Phase 3 trials may shift compounds from L2 animal studies to L3 human trials. New FDA approvals may shift L3 human trials to L4 FDA-approved. PSI's Editorial Standards review cadence updates compound-level evidence designations as primary-source evidence becomes available. The methodology framework itself is stable while individual compound designations are dynamic. FDA Drug Approval Process, ICH E6 Good Clinical Practice, GRADE methodology, USPSTF Procedure Manual, Cochrane Handbook, and AHRQ Evidence-Based Practice Center frameworks remain foundational. PSI tracks methodology updates per the Editorial Standards review cadence.

Comparison

Tier	Visitor Label	Evidence Scope	Example Compounds
L1	Preclinical	In vitro, mechanistic, computational modeling	Early structure-activity work, novel research peptides without animal data
L2	Animal Studies	Rodent and other animal models, no human data	Compounded BPC-157, TB-500, epitalon, many emerging research peptides
L3	Human Trials	Pilot, Phase 1, Phase 2, Phase 3 (no FDA approval)	Compounded thymosin alpha-1 (international Zadaxin approval), cerebrolysin, retatrutide in development
L4	FDA Approved	Completed FDA pre-market review and approved label	Wegovy, Ozempic, Zepbound, Mounjaro, Forteo, Tymlos, Evenity, Tesamorelin, Vyleesi
Cross-tier	Highest tier	Highest tier with substantive evidence	Per Editorial Standards disambiguation rules with primary-source citations
Off-label	Per AMA Code 1.1.5	FDA-approved compound used outside FDA label indication	Off-label use of L4 compound operates under AMA Code 1.1.5 framework
International	Reflected as L3	International approvals not equivalent to US FDA approval	Zadaxin thymosin alpha-1 (35-country approval) classified L3 human trials
Research-grade	Outside framework	Research chemical labeled not for human use	Operates outside framework and outside validated clinical practice entirely

Who This Applies To

· Patient reading PSI compound pages and seeking framework context for evidence-level labels.
· Physician evaluating peptide therapy evidence quality for shared decision-making with patients.
· Researcher reviewing PSI methodology for evidence classification consistency with published frameworks.
· Patient evaluating FDA-approved versus compounded peptide pathway distinctions for indication match.
· Adult considering compounded peptide therapy and reviewing evidence base limitations transparently.
· Physician seeking AMA Code of Medical Ethics 1.1.5 risk-benefit assessment framework for off-label prescribing.
· Patient comparing peptide therapy options across condition with mixed evidence-tier compound classes.
· Adult evaluating peptide therapy claims against framework definitions for honest communication standards.
· Researcher seeking GRADE, USPSTF, Cochrane, or AHRQ framework cross-reference for PSI methodology.
· Patient seeking transparent communication of peptide therapy evidence quality for informed decision-making.

Verdict

PSI uses a transparent four-tier evidence framework. The framework distinguishes preclinical, animal studies, human trials, and FDA-approved compounds. Visitor-facing labels are FDA Approved, Human Trials, Animal Studies, and Preclinical. The framework is anchored in established evidence-hierarchy methodology. The framework supports transparent claim communication and physician archetype trust. Every compound on PSI declares one locked evidence level traceable to peer-reviewed primary sources. Cross-tier disambiguation rules apply for compounds spanning tiers. The Editorial Standards page documents the full methodology. The framework is permanent infrastructure protecting PSI's communication credibility.

In Plain Terms

PSI grades peptide research using four tiers. Tier 1 covers lab studies in cells and computer models. Tier 2 covers animal studies in mice and other animals. Tier 3 covers human trials including small pilot studies and large Phase 3 trials. Tier 4 covers FDA-approved drugs. Every compound on PSI has one declared tier. Lab studies tell us how something might work. Animal studies tell us what happens in a living organism. Human trials tell us what happens in actual patients. FDA approval is the strongest evidence. The Editorial Standards page explains the full framework.

Evidence comes in different levels. Lab studies show how something works at a cellular level. Animal studies show what happens in living mice or other animals. Human trials show what happens in actual patients. FDA approval is the gold standard for safety and effectiveness. PSI tells you which level applies to each peptide. The labels say FDA Approved, Human Trials, Animal Studies, or Preclinical. Higher levels mean stronger evidence. The Editorial Standards page explains the full system.

Evidence-tier framework supports communication of peptide therapy evidence quality across the PSI compound library. The framework does not substitute for physician selection through state medical board license verification, ABMS board certification, and AMA Code 1.1.5 documentation practice. PSI maintains a vetted directory of practitioners ordering comprehensive baseline bloodwork and applying evidence-tier framework to prescribing decisions.

Find a verified physician

PSI's directory only lists physicians who have passed a five-gate verification process: state board active, no disciplinary actions, peptide-category competency, transparent pricing, and patient outcome documentation.

Browse the directory Learn about the verification process →

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Common Contexts

· Patient reading compound pages and seeking framework context for evidence-level labels
· Physician evaluating peptide therapy evidence quality for shared decision-making
· Researcher reviewing PSI methodology for evidence classification consistency
· Patient evaluating FDA-approved versus compounded peptide pathway distinctions
· Adult considering compounded peptide therapy and reviewing evidence base limitations
· Physician applying AMA Code 1.1.5 risk-benefit assessment to off-label prescribing
· Patient comparing peptide therapy options across mixed evidence-tier compound classes
· Adult evaluating peptide therapy claims against framework definitions for honest standards
· Researcher seeking GRADE, USPSTF, Cochrane framework cross-reference for PSI methodology
· Patient seeking transparent evidence quality communication for informed decision-making

Important Context

This page is educational and does not constitute medical advice. The information presented reflects PSI's Editorial Standards documenting the four-tier evidence framework methodology. The methodology is anchored in established evidence-hierarchy frameworks including GRADE (Grading of Recommendations Assessment, Development, and Evaluation), USPSTF Procedure Manual, Cochrane Handbook for Systematic Reviews of Interventions, AHRQ Evidence-Based Practice Center framework, FDA Drug Approval Process under 21 CFR 312 (IND) and 21 CFR 314 (NDA), ICH E6 Good Clinical Practice guidance, and AMA Code of Medical Ethics 1.1.5 for off-label and compounded prescribing context.

The evidence-tier framework supports communication of peptide therapy evidence quality. The framework does not substitute for individualized clinical judgment regarding your specific patient context. Specialty coordination across endocrinology, weight medicine, rheumatology, sports medicine, immunology, infectious disease, women's health, and men's health supports comprehensive evidence interpretation. AMA Code of Medical Ethics 1.1.5 framework governs off-label and compounded prescribing decisions including documented risk-benefit assessment and FDA-approved alternatives considered.

Self-interpretation of evidence-tier framework is not a substitute for physician clinical judgment in your specific situation. Compounds at any evidence tier require physician prescription and clinical oversight. Research-grade peptide products labeled not for human use operate outside the evidence-tier framework entirely and outside the validated clinical practice framework.

Educational content only. The four-tier evidence framework supports transparent communication of peptide therapy evidence quality. Dosing should be determined by a qualified physician who can evaluate your individual situation. PSI does not provide personalized clinical recommendations. Discuss with your physician before pursuing any peptide therapy. For compounded preparations, AMA Code 1.1.5 framework applies.

Sources and Citations

[1] FDA Drug Approval Process: New Drug Application (NDA) and Biologics License Application (BLA) framework under 21 CFR 312 (IND) and 21 CFR 314 (NDA) · US Food and Drug Administration · 2024 · Source
[2] ICH E6 Good Clinical Practice: International Council for Harmonisation guideline for clinical trial design, conduct, monitoring, recording, analysis, and reporting · International Council for Harmonisation · 2023 · Source
[3] GRADE Handbook: Grading of Recommendations Assessment, Development, and Evaluation framework for systematic review and clinical practice guideline development · GRADE Working Group · 2024 · Source
[4] USPSTF Procedure Manual: US Preventive Services Task Force methodology for clinical preventive services recommendations · US Preventive Services Task Force · 2024 · Source
[5] Cochrane Handbook for Systematic Reviews of Interventions: methodology for systematic reviews and meta-analyses of healthcare interventions · Cochrane Collaboration · 2024 · Source
[6] AHRQ Evidence-Based Practice Center framework: Agency for Healthcare Research and Quality methodology for evidence-based practice center reports · Agency for Healthcare Research and Quality · 2024 · Source
[7] Lincoff AM, Brown-Frandsen K, Colhoun HM, et al. Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes (SELECT trial) · New England Journal of Medicine · 2023 · DOI
[8] Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1) · New England Journal of Medicine · 2022 · DOI
[9] Marso SP, Bain SC, Consoli A, et al. Semaglutide and Cardiovascular Outcomes in Patients with Type 2 Diabetes (SUSTAIN-6) · New England Journal of Medicine · 2016 · DOI
[10] Neer RM, Arnaud CD, Zanchetta JR, et al. Effect of Parathyroid Hormone (1-34) on Fractures and Bone Mineral Density in Postmenopausal Women with Osteoporosis (VERT trial) · New England Journal of Medicine · 2001 · DOI
[11] FDA Prescribing Information: Wegovy (semaglutide) injection · 2024 · FDA NDA 215256 · Source
[12] FDA Prescribing Information: Zepbound (tirzepatide) injection · 2024 · FDA NDA 217806 · Source
[13] FDA Prescribing Information: Forteo (teriparatide) injection · 2020 · FDA NDA 021318 · Source
[14] FDA Prescribing Information: Evenity (romosozumab-aqqg) injection with cardiovascular boxed warning · 2019 · FDA NDA 761062 · Source
[15] 21 CFR Part 312: FDA Investigational New Drug (IND) framework regulation · US Code of Federal Regulations · 2024 · Source
[16] 21 CFR Part 314: FDA New Drug Application (NDA) framework regulation · US Code of Federal Regulations · 2024 · Source
[17] AMA Code of Medical Ethics Opinion 1.1.5: Off-label and Investigational Use of Pharmaceuticals · American Medical Association · 2024 · Source
[18] ClinicalTrials.gov: NIH database of privately and publicly funded clinical studies conducted around the world · National Library of Medicine, NIH · 2024 · Source

Medical Disclaimer

This content is for educational and informational purposes only and does not constitute medical advice. The information presented reflects published research as indexed by PSI and should not be used to make treatment decisions. Always consult a qualified healthcare provider before starting, stopping, or modifying any treatment.