Phase II

Once the initial safety of the therapy has been confirmed in Phase I trials, Phase II trials are performed on larger groups (20-300) and are designed to assess clinical efficacy of the therapy; as well as to continue Phase I assessments in a larger group of volunteers and patients. The development process for a new drug commonly fails during Phase II trials due to the discovery of poor efficacy or toxic effects.

Phase II studies are sometimes divided into Phase IIA and Phase IIB. Phase IIA is specifically designed to assess dosing requirements, whereas Phase IIB is specifically designed to study efficacy.

Some trials combine Phase I and Phase II into a single trial, monitoring both efficacy and toxicity.