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What are the phases of a clinical trial?

The ‘phase’ of a clinical trial refers to how much is known about the new treatment/test.

  • A Phase I trial is the first time a treatment is tested in humans. Phase I trials usually involve a small number of healthy volunteers or patients and a range of doses to test how the body responds to the treatment and to look for side effects.
  • A Phase II trial tests the effects of a drug in people with a particular disease. These trials usually involve small numbers of patients and are used to work out the correct dose of the treatment and how effective it is in the short term.
  • A Phase III trial compares the effect of a new treatment with the current standard treatment and usually involves large numbers of patients in several countries.
  • A Phase IV trial is a trial or study of a drug that is already approved. Phase IV studies check how well a treatment works over a longer period of time and monitor long-term side effects.