adverse event

adverse event (plural adverse events)

1. (health sciences, medicine) Any occurrence that has any negative (deleterious) influence (in any degree from mild to severe).

   Hyponyms: adverse effect (loosely synonymous), side effect (loosely synonymous)

   adverse events in the treatment group

   adverse events in the control group

In precise usage, some writers preserve the distinction that an event that happens during a scientific study either may or may not be an effect (as in cause and effect) of the treatment being tested (correlation does not imply causation); thus they prefer the term adverse event generally and reserve the term adverse effect for known or likely causation. But in imprecise usage the two terms are often used synonymously.