Severe daytime somnolence in patients treated with an MAOI

Eight patients with hypersomnolent, anergic major depression benefited markedly from treatment with relatively high doses of phenelzine or tranylcypromine but experienced intense afternoon somnolence and disrupted sleep. Reducing the dose of monoamine oxidase inhibitor (MAOI) or substituting isocarboxazid sometimes provided relief, but altering the schedule of drugs or meals did not. Bedtime sedation alleviated the disrupted sleep but had little effect on daytime somnolence. The mechanism underlying this side effect is unknown; sleep deprivation, narcolepsy, or hypotension does not account for it. Patients given an MAOI should be assessed for this disturbance and cautioned to avoid risk of injury when it occurs.