An adverse event (AE) is any adverse change in health or "side-effect" that occurs in a person who participates in a clinical trial while the patient is receiving the treatment (study medication, application of the study device, etc.) or within a pre-specified period of time after their treatment has been completed.

AEs in patients participating in clinical trials must be reported to the local Institutional Review Board (IRB) and the study sponsor. Adverse events categorized as "serious" (for example death, illness requiring hospitalization, events deemed life-threatening, or involving cancer or fetal exposure) must be reported to the regulatory authorities immediately, whereas minor adverse events are merely documented in the annual summary sent to the regulatory authority.

The sponsor collects AE reports from the local researchers, and notifies all participating sites of the AEs at the other sites, as well as both the local investigators' and the sponsors' judgment of the seriousness of the AEs. This process allows the sponsor and all the local investigators access to a set of data that might suggest potential problems with the study treatment while the study is still going on.