Involve the initial introduction of an investigational new drug into humans. Phase I trials are closely monitored and may be conducted in patients or in healthy volunteers. The studies are designed to determine the metabolism and pharmacologic actions of the drug in humans, safety, side effects associated with increasing doses, and if possible, early evidence of effectiveness. The trials also can include studies of structure-activity relationships, mechanisms of action in humans, use of the investigational drug as research tools to explore biological phenomena, or disease processes. The total number of patients included in Phase I studies varies but is generally in the range of 20 to 80. Sufficient information should be obtained in the trial to permit design of well-controlled, scientifically valid Phase II studies.

In Phase I a trial tests a potential new treatment with a small number of volunteers (20-80 individuals) to determine the treatment's safety and potential side effects. Participants in Phase I trials may include patients and "healthy" participants.

The first phase of a clinical trial. Generally, Phase I studies use drugs that have never been used in humans before. The goal is to obtain the maximum tolerated dose and to gain early evidence of efficacy. A small number of participants are used in this phase.