Definition
Clinical development proceeds in phases after preclinical work. Phase 1 trials, usually in small groups of healthy volunteers, assess safety, tolerability and pharmacokinetics. Phase 2 trials, in up to a few hundred participants with the target condition, explore preliminary efficacy and refine the study design. Phase 3 trials enrol hundreds to thousands of participants, often in randomised controlled designs, to confirm efficacy and characterise safety before a regulatory submission. Phase 4 refers to monitoring after approval. Many compounds fail at each stage, most often in phase 2 for lack of efficacy. A compound described as in clinical trials has not necessarily shown benefit; the phase and published results determine what is known.