Why are Phase 4 studies conducted after a drug reaches the market?

Phase 4 studies are conducted after a drug has reached the market to focus on collecting safety data from a much larger population of patients than was possible during the initial clinical trial phases.1

While Phase 1, 2, and 3 trials involve several hundred to several thousand participants, they may not be large enough to identify rare but serious side effects that occur infrequently—for example, an event that might only affect one in 10,000 people. By monitoring the drug's performance in the general population, regulators and pharmaceutical companies can better identify these unforeseen adverse drug events that were not apparent during pre-marketing testing.12

This post-marketing surveillance is a critical component of drug safety, as it allows for the ongoing evaluation of a medication's risk-benefit profile once it is widely available for use.2