Phase II trials investigate a drug in patients, rather than in the healthy subjects tested in Phase I trials. This phase continues to provide safety data as the number of people receiving the drug increases. However the focus of these studies is the efficacy of the drug, and the intention is to prove the hypothesis of how the drug works.

Phase II trials can be further divided into Phase IIa and Phase IIb studies. Phase IIa studies focus on proving the hypothesized mechanism of action – often called ‘proof-of-concept’ or ‘proof-of-mechanism’ studies – and tend to be with fewer numbers of patients than Phase IIb studies. Having confirmed the hypothesis, Phase IIb studies look to find the optimum dose at which the drug shows biological activity with minimal side-effects – often called ‘dose-ranging’ studies.