Many terms are used to describe harms associated with healthcare interventions. This can confuse review authors, particularly as published papers often use terms loosely and interchangeably. Some common related terms include ‘adverse event’ (an unfavourable outcome that occurs during or after the use of a drug or other intervention but is not necessarily caused by it), ‘adverse effect’ (an adverse event for which the causal relation between the intervention and the event is at least a reasonable possibility), ‘adverse drug reaction’ (an adverse effect specific to a drug), ‘side effect’ (any unintended effect, adverse or beneficial, of a drug that occurs at doses normally used for treatment), and ‘complications’ (adverse events or effects following surgical and other invasive interventions).