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Phase 1, 2, 3, 4 trials


Different stages of clinical testing of a new drug.

  • Phase 1: First testing in humans, primarily to test safety. A drug is given to a small number of healthy volunteers who are closely monitored.
  • Phase 2: Testing in a small number of patients, to assess safety, to monitor how a drug is metabolized, and to gather initial data on efficacy.
  • Phase 3: A large trial in patients to test efficacy and safety. Pivotal phase 3 trials (or registration trials) provide the key data on efficacy in submissions for regulatory approval.
  • Phase 4: Studies undertaken after a drug has been licensed, to gather further safety, efficacy or effectiveness data in routine clinical use.

Phase 2 studies are sometimes divided into phase 2A and 2B. There are no formal definitions of these divisions, but phase 2A studies are typically more preliminary and can address issues such as dosing and safety, while phase 2B studies are generally ‘mini-phase 3’ studies that provide data on efficacy.

Increasingly, these categories are becoming less clear-cut, particularly phase 2 and 3 trials.