The broadest of these terms is adverse drug event or ADE which covers not only adverse drug reactions but any errors from prescribing or administering drugs. The original World Health Organisation definition of an adverse drug reaction as opposed to event is a response to a drug which is noxious and unintended and which occurs at doses normally used for prophylaxis, diagnosis or therapy of disease. (WHO 1970). A more recent definition is:

An appreciably harmful or unpleasant reaction, resulting from an intervention related to the use of a medicinal product, which predicts hazard from future administration and warrants prevention or specific treatment, or alteration of the dosage regimen, or withdrawal of the product. (Edwards and Aronson 2000 p. 1255)

A side effect of a medicine might be defined as any result that occurs in addition to the intended effect. Some side effects can be useful. A side effect of the antihistamine diphenhydramine hydrochloride is drowsiness; so if you have a rash that itches and you take it at night, it may help you sleep through the itch and help prevent you scratching. If however you take the same antihistamine for itch in the daytime and drive into a tree, the drowsiness was an unintended side effect that proved noxious at the correct dose. ADRs, therefore, are always adverse, while side effects might be useful. Adverse drug reactions aren’t the same as overdoses because, looking at the definition again, ADRs occur at normal doses. An overdose could be covered by the term Adverse Drug Event.