Which phase of drug approval primarily involves safety testing on humans?

The first phase of drug approval, known as Phase I, primarily focuses on safety testing in humans. During this phase, a small group of healthy volunteers is administered the drug to determine its safety profile, including how the drug is absorbed, metabolized, and excreted by the body. The goal is to identify any potential side effects and to establish the appropriate dosage range.

This phase is crucial because it marks the transition from preclinical trials, which often involve laboratory and animal studies, to human trials. The insights gathered during Phase I help researchers understand how the drug behaves in the human body and whether it poses any risks to patients.

Subsequent phases, such as Phase II and III, build on this initial safety data but also include efficacy testing in larger populations with the condition the drug is intended to treat, while Phase IV is concerned with post-marketing surveillance of the drug's long-term effects and performance in the general population. Thus, it is Phase I that specifically concentrates on assessing the safety of a drug with human subjects.