Monoamine oxidase inhibitors — MAOIs — are the oldest class of antidepressants and, in the right hands, among the most effective. They are also the class that asks the most of the patient: dietary restrictions, drug interaction avoidance, washout periods, and a willingness to live with a medication that has serious consequences for ignoring instructions. That tradeoff is why MAOIs today belong almost entirely to specialty psychiatric practice.
- Class
- Monoamine Oxidase Inhibitors
- Mechanism
- Inhibit MAO-A and/or MAO-B → prevent breakdown of dopamine, norepinephrine, serotonin → elevated synaptic monoamines
- Typical dose
- Phenelzine 45-90 mg/day; tranylcypromine 30-60 mg/day; selegiline patch 6-12 mg/24h
- Half-life
- Irreversible inhibition — clinical effect persists ~2 weeks after discontinuation as new MAO regenerates
- FDA indications
- MDD (particularly treatment-resistant or atypical features)
- Key adverse effects
- Hypertensive crisis with tyramine ingestion (aged cheeses, cured meats, fermented foods, certain wines, fava beans), serotonin syndrome with serotonergic drug combinations, orthostatic hypotension, weight gain, sexual dysfunction
- Representative agents
- Phenelzine, tranylcypromine, isocarboxazid, selegiline (transdermal at higher doses non-selective)
Black box: Suicidal thinking/behavior in pediatric and young adult patients
Modern niche: treatment-resistant depression after multiple failed trials. Requires dietary restrictions, washout from other antidepressants (especially fluoxetine — 5 week washout), careful drug interaction screening. Specialty psychiatry generally required.
The story begins with serendipity. In the 1950s, patients with tuberculosis being treated with isoniazid — an antibacterial that incidentally inhibits monoamine oxidase — were noticed by ward physicians to be unexpectedly cheerful. That observation seeded a research program that gave rise to the monoamine hypothesis of depression and the first deliberately marketed antidepressants. Iproniazid and its successors established that drugs which raise brain monoamines treat depression. Everything since — TCAs, SSRIs, SNRIs, atypicals — descends from that original insight.
The mechanism is simple. Monoamine oxidase is the enzyme that degrades dopamine, norepinephrine, and serotonin. MAO comes in two flavors — MAO-A, which prefers serotonin and norepinephrine, and MAO-B, which prefers dopamine. Classic MAOIs — phenelzine, tranylcypromine, isocarboxazid — inhibit both irreversibly. The transdermal selegiline patch at higher doses also becomes non-selective. The effect is sustained elevation of all three monoamines.
MAOIs raise all three monoamines by blocking their degradation — a powerful but logistically demanding mechanism with the tyramine interaction as its defining hazard.
Mechanism note: MAOIs are genuinely effective where other antidepressants fail, but the tyramine reaction and serotonin-syndrome interactions demand a disciplined prescriber and an educated patient.
The catch is that MAO doesn't just operate in the brain. Intestinal MAO degrades dietary tyramine before it reaches the systemic circulation. With MAO inhibited, tyramine from aged cheeses, cured meats, fermented foods, draft beer, fava beans, and certain wines enters the bloodstream and displaces norepinephrine from sympathetic storage vesicles. The result is a hypertensive crisis — severe headache, BP often above 200 systolic, risk of intracranial hemorrhage. The "cheese reaction" is what made MAOIs feared, and the dietary restrictions are absolute, not approximate.
The drug interaction side is equally serious. Combining an MAOI with any serotonergic drug — SSRI, SNRI, tramadol, dextromethorphan, meperidine, linezolid (an antibiotic that is also a weak MAOI), St. John's Wort — can produce fatal serotonin syndrome. The washout from a serotonergic agent before starting an MAOI is two weeks for most agents and five weeks for fluoxetine because of its long half-life. Starting an MAOI requires reviewing every medication the patient takes.
Given all this, why use them? Because they work. In treatment-resistant depression, after multiple failed trials of safer agents, MAOIs achieve responses that nothing else does. They are particularly effective in atypical depression — the phenotype with mood reactivity, hypersomnia, hyperphagia, and rejection sensitivity. For the right patient who has tried everything else and is willing to do the work, an MAOI can be transformative.
The patient who takes an MAOI signs up for a partnership. They learn the food list. They alert every prescriber to the medication. They carry information about emergency management. They accept washout periods when transitioning. In return, they sometimes get back a level of function that no other antidepressant gave them.
MAOIs are an old class. They are not obsolete. They are highly specialized — and the patient who needs one deserves a clinician who knows how to prescribe one carefully.