Before a new drug or treatment is approved for public use, it must pass through a structured series of tests, each one designed to answer a specific question. Is it safe? Does it work? How well does it work across a broad population? Understanding this process is essential for interpreting the evidence behind any compound.
Before it all: Pre-clinical Testing
All compounds first undergo laboratory (in vitro) and animal studies before any human involvement. Only compounds that show a meaningful safety and efficacy signal in pre-clinical testing move forward to clinical trials. The vast majority never do.
Survival Rate
The Drug Attrition Funnel
Of all compounds that enter Phase 1 clinical trials, only a small fraction survive all three phases to reach regulatory approval.
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Percentages are approximate and vary significantly by therapeutic area. Sources: FDA Drug Development Process; StatPearls NBK546595.
Phase 1
Safety & Dosage
Primary question: Is this safe? Researchers administer the compound to a small group, typically healthy volunteers, and monitor closely how the body processes it, what side effects emerge as the dose is increased, and the maximum tolerated dose.
For certain diseases such as oncology, healthy volunteers cannot ethically be enrolled, so Phase 1 recruits patients with the target condition instead. About 70% of compounds clear this phase.
Phase 2
Efficacy & Side Effects
Primary question: Does it work? Researchers now enroll participants who have the target disease or condition. The trial evaluates whether the compound produces a meaningful clinical effect, while continuing to monitor safety and short-term side effects.
Many Phase 2 trials include a control arm (either placebo or standard-of-care) for direct comparison. Phase 2 has the steepest attrition: only about one in three compounds that pass Phase 1 show sufficient efficacy to justify the far larger investment of Phase 3.
Phase 3
Scale & Confirmation
Primary question: How well does it work, and for whom? A large, diverse participant pool allows researchers to confirm efficacy, uncover less common side effects that smaller trials would miss, and compare the treatment head-to-head against the current standard of care.
Successful completion of Phase 3 is the basis for a regulatory submission, specifically a New Drug Application (NDA) to the FDA in the US. If approved, the developer gains the right to market the drug to the public.
Regulatory Review
If Phase 3 succeeds, the developer submits a complete dossier of trial data to regulatory agencies (the FDA in the US, the EMA in Europe). Reviewers independently evaluate all evidence before granting approval to market the drug.
Phase 4
Post-Market Surveillance
Primary question: What happens in the real world? Even rigorous Phase 3 trials involve a finite, selected population. Phase 4 monitors approved drugs across the full diversity of real-world patients (different ages, comorbidities, and drug combinations) over extended timespans.
Rare adverse events affecting 1 in 10,000 patients may only surface at this scale. If serious unforeseen risks emerge, regulatory agencies can issue warnings, add label restrictions, or withdraw the drug from the market entirely.
Context for this site
What this means for research peptides
The vast majority of compounds profiled on Peptide Commons have not completed Phase 3 human trials. Most exist in a pre-approval state, with evidence ranging from pre-clinical animal data to small Phase 1 or Phase 2 human studies.
When a product page notes "preliminary animal evidence" or "limited Phase 2 data," this framework is what that refers to. Knowing where a compound sits in this pipeline is essential context for evaluating its evidence base.
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