Phase I trials are the first stage of testing in human subjects, normally performed with a small group of healthy volunteers. This phase includes trials designed to assess the safety, tolerability, pharmacokinetics, and pharmacodynamics of a candidate drug.

In order to first test a candidate drug in humans, the pharmaceutical company responsible for the trial must obtain permission from the regulatory agency in each country in which the study is to be conducted. The application dossier includes all available information regarding the candidate drug, and is known as the Investigational New Drug Application (IND) in the USA, and the Clinical Trial Application (CTA) in the EU. The decision to grant permission to conduct a clinical trial is based on the preclinical data regarding safety, efficacy, pharmacology, and toxicology in animal models.