The efficacy of antidepressants is measured primarily by their ability to treat the acute symptoms of depression yet the underlying process may take longer to resolve. A period of emotional frailty follows resolution of the acute stage, during which the patient, although apparently symptom-free, is prone to suffer relapse of the original symptoms. Antidepressants should not be stopped immediately a response is observed, but should be given for a longer period to prevent early relapse. In those suffering from recurrent depression, long-term treatment may prevent later recurrences of new episodes of depression.

The distinction between relapse of old symptoms and recurrence of new episodes is of particular importance, both in assessing efficacy of antidepressants during the continuation phase of acute treatment and in the separate assessment of their prophylactic efficacy. Unfortunately, though, many investigations have failed to distinguish between these two distinct phenomena. Klerman & Paykel (1970) emphasised the need to distinguish between early return of depressive symptoms, which they label ‘relapses’, from the later new episodes which they term ‘recurrence’. However, there has been a general lack of systematic investigation of this question, as well as a failure to use sufficiently precise methodology to distinguish between relapse and recurrence. Some studies which purported to be of prophylaxis were in effect only examining the continuation phase of acute treatment.