adverse event

Trials count adverse events as their safety measure, and the count is broader than it looks. The international guideline that standardised the term, ICH E2A, defines an adverse event as any untoward medical occurrence in a patient or clinical investigation subject administered a pharmaceutical product, which does not necessarily have a causal relationship with the treatment. It covers any unfavourable and unintended sign, symptom, disease or abnormal laboratory finding that occurs while someone is taking the product, whether or not it is considered related to it. An event judged to carry at least a reasonable possibility of having been caused by the drug has a different name: an adverse drug reaction.

Serious is a category, not a measure of strength. ICH E2A separates serious from severe, which describes only how intense an event was, and notes that a severe event such as a severe headache may be of relatively minor medical significance. An adverse event is serious when it results in death, is life-threatening, requires inpatient hospitalisation or prolongs an existing stay, results in persistent or significant disability or incapacity, or is a congenital anomaly or birth defect. The United States Food and Drug Administration lists the same outcomes and adds other important medical events that may jeopardise the patient or may require intervention to prevent one of them.

So a trial reporting the same share of serious adverse events in both of its groups is reporting what happened to the people in them, not what the drug did. Reading such a count means knowing which outcomes it counted, and that events nobody attributes to the treatment are counted in it too.