any undesirable, unexpected medical occurrence that happens during or after the administration of a medication, treatment, or procedure, but is not necessarily caused by it. Adverse events can be mild or severe and can occur immediately or develop over time.
Synonyms: Complications, Negative Reactions, Undesirable Events, Unintended Events
Example:
May include nausea, headache, dizziness, allergic reactions, organ damage, or even death.