Clinical Trial Phases Explained

What happens in each phase of a clinical trial, from preclinical studies through Phase IV post-marketing surveillance.

Filed under evidence research, written for beginner, patient-consumer, researcher, investor.

Not medically reviewed. Last updated 2026-03-15. Nothing here is medical advice.

Clinical trials follow a structured pathway to evaluate whether a new drug is safe and effective. For peptide drugs, this process typically takes 10–15 years from discovery to approval.

Preclinical Stage

Before human testing, peptides are studied in:

• Laboratory (in vitro) experiments
• Animal models (in vivo)
• Toxicology assessments
• Pharmacokinetic studies

Phase I — Safety and Dosing

• First-in-human studies
• Typically 20–80 healthy volunteers
• Primary goal: safety, tolerability, pharmacokinetics
• Duration: several months

Phase II — Efficacy and Side Effects

• Tests the drug in patients with the target condition
• Typically 100–300 patients
• Primary goal: efficacy and optimal dosing
• Duration: months to 2 years

Phase III — Confirmation

• Large-scale trials to confirm efficacy and monitor adverse reactions
• Typically 1,000–3,000+ patients
• Compared against placebo or standard treatment
• Required for regulatory approval
• Duration: 1–4 years

Regulatory Review

After successful Phase III trials, the manufacturer submits a New Drug Application (NDA) to the FDA or equivalent application to other agencies. Review typically takes 6–12 months.

Phase IV — Post-Marketing Surveillance

After approval, ongoing monitoring tracks:

• Rare adverse events
• Long-term safety
• Use in special populations
• Real-world effectiveness

This guide explains the clinical trial process and does not evaluate specific trials or products.

More guides: All guides, Safety centre, Peptide library.