Clinical trials are used to assess the safety of new medicines. Pharmacovigilance is the process of collecting safety data after the drug has been approved for use and giving this information to the regulatory authorities. The data collected includes adverse effects associated with use of a drug.

Pharmacovigilance is required after marketing approval to detect potential adverse reactions that could be due to use of the medicine. In particular, it can detect rare side effects that are not picked up during clinical trials or affect specific groups of individuals.

A phase 4 clinical trial investigates the occurrence of side effects over time by a new treatment after it has been approved and is on the market.