The standardized stages of human drug testing — each phase answers different questions and involves more people than the last.Phase I: Is this safe? Conducted in 20–100 healthy volunteers or patients. Establishes safety, tolerability, and dosing. Phase II: Does it work, and at what dose? 100–300 patients in the target population. Generates efficacy signals. Phase III: Does it work definitively, at scale? 300–3,000+ patients in randomized controlled trials against placebo or standard of care. Required for regulatory approval. Phase IV: Post-marketing surveillance after approval. A compound that has only completed Phase I has established basic safety — not efficacy.