Adverse event.

An unwanted medical occurrence during use of a compound.

An adverse event is any unfavourable medical occurrence in someone using a compound, whether or not it is judged to be caused by that compound. The wording is deliberate: in formal trial reporting, everything that happens is recorded, and causation is assessed separately, because deciding in advance which events count would bias the safety picture. This is why adverse event lists include things that plainly had nothing to do with the treatment, and why the honest comparison is always against the control group rather than against zero. A serious adverse event is a defined subcategory covering outcomes such as death, hospitalisation, or lasting disability, and it is a formal classification rather than a description of how unpleasant something felt. This vocabulary matters in peptide discussion because compounds that have never been through structured trials have no adverse event data at all, only scattered self-report. The absence of a documented problem in that situation is not evidence of safety; it reflects the absence of systematic collection. The misunderstanding worth correcting runs in both directions: an adverse event in a trial does not prove the drug caused it, and no reported adverse events for an untested compound does not mean none occur.