Phase I

Also known as “first-in-humans” study, is the initial stage of clinical trials. It primarily tests the safety, side effects, optimal dose, and formulation method of a new drug in a small group of 20-100 healthy volunteers. The study includes dose-ranging or dose escalation to find the safest dose and to identify any toxic levels. The trials are usually conducted in clinical trial clinics and are not randomized. The participants are observed until several half-lives of the drug have passed. In some cases, patients with terminal illnesses like cancer or HIV may participate. The study is followed by further phases (II, III, IV) if successful.

Source: Wikipedia

Not to be confused with: Phase 0 trial
Not to be confused with: Preclinical trial