adverse event

In a clinical trial, an adverse event is any unfavourable change in a participant's health during the study or for a set period after it, including abnormal laboratory results. The definition used by ClinicalTrials.gov, the trial registry run by the United States National Library of Medicine, is deliberately wide: the change may or may not have been caused by the treatment being tested. A broken wrist, a chest infection or a rise in a liver enzyme all count, whatever brought them about.

That width is why a raw count of adverse events is not a count of harms. Everything is written down in every group, including the group taking a dummy tablet, so the number that carries meaning is the comparison between the groups rather than the total on the drug. A side effect shows up as an imbalance: more of one kind of event among the people taking the treatment than among those taking placebo.

Trials report the events that meet a formal threshold separately, as serious adverse events. Serious there is a regulatory category rather than a word about how unpleasant something felt, so a painful but self-limiting problem may be recorded as not serious, while a symptomless finding that puts someone in hospital is serious.