Which phase of clinical testing typically involves the largest number of volunteers?

Phase 3 clinical testing is typically characterized by the involvement of the largest number of volunteers. This phase is crucial for determining the efficacy and safety of a drug in a larger population and often includes hundreds to thousands of participants. The expansive participant pool helps researchers gather more comprehensive data on the drug's effectiveness and monitor a broader range of potential side effects across diverse demographics.

In contrast, Phase 1 testing generally includes a smaller group of healthy volunteers and primarily focuses on safety, dosage, and pharmacokinetics. Phase 2 expands Research to a larger group, usually involving individuals with the condition the drug is intended to treat, but still does not reach the sample size seen in Phase 3. Phase 4, also known as post-marketing surveillance, occurs after a drug has been approved for market use and focuses on long-term effects and broader population use but is not solely a clinical testing phase like the previous phases. Hence, the substantial participant involvement in Phase 3 makes it a critical part of the drug development process.