Step 1 topics → Psychiatry → Monoamine Oxidase Inhibitors (MAOIs)

Monoamine Oxidase Inhibitors (MAOIs)

USMLE Step 1 trap: Confuses MAO-A and MAO-B selectivity and fails to distinguish selegiline from nonselective MAOIs. Phenelzine and tranylcypromine are nonselective (A+B), selegiline preferentially inhibits MAO-B at low doses (used in Parkinson's), and MAO-A is the isoform critical for tyramine and serotonin metabolism.

MAOIs are among the oldest antidepressants and still show up on USMLE Step 1 — not because you'll prescribe them often, but because they generate high-yield pharmacology questions around mechanism, dangerous interactions, and management principles. The exam tests this topic from three angles: understanding MAO-A vs MAO-B isoform selectivity (and which agents target which), interpreting a clinical scenario involving tyramine-rich food and acute hypertension, and calculating the correct washout period when switching between MAOIs and serotonergic drugs. These aren't just recall questions — several stems drop you into a passage and ask you to identify the crisis type or select the safest next step.

The concept is tricky because students conflate two very different crises: serotonin syndrome and tyramine-induced hypertensive emergency. They look similar on the surface (patient on an MAOI, something goes wrong), but the mechanisms, triggers, and treatments are completely different. Serotonin syndrome is a serotonergic excess state treated with cyproheptadine; tyramine crisis is a catecholamine surge treated with alpha-blockers. Mixing these up on a clinical vignette is a common and costly mistake.

The washout period question catches students off guard because most assume it's symmetric — same wait time no matter which drug you stop. It's not. Fluoxetine's unusually long half-life (and active metabolite norfluoxetine) means it requires a 5-week washout before starting an MAOI, while other SSRIs only need 2 weeks. USMLE Step 1 will absolutely test this asymmetry, usually by specifying the SSRI in the stem and asking how long to wait.

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Common misconceptions

Common mistake
Wrong: All MAOIs inhibit both MAO-A and MAO-B equally and are interchangeable.
Right: Phenelzine and tranylcypromine are nonselective (A+B), selegiline preferentially inhibits MAO-B at low doses (used in Parkinson's), and MAO-A is the isoform critical for tyramine and serotonin metabolism.
Selegiline is not interchangeable with phenelzine or tranylcypromine. At low therapeutic doses, selegiline selectively inhibits MAO-B, which is why it's used in Parkinson's disease (MAO-B degrades dopamine in the striatum). Phenelzine and tranylcypromine are nonselective and hit both isoforms. The distinction matters clinically because MAO-A is the isoform responsible for degrading tyramine and serotonin — nonselective MAOIs carry the full tyramine crisis risk, while low-dose selegiline has less dietary restriction concern.
Common mistake
Wrong: The tyramine crisis with MAOIs is a serotonergic reaction.
Right: Tyramine crisis is a hypertensive emergency caused by uninhibited sympathomimetic action of tyramine (normally degraded by gut/liver MAO-A), treated with phentolamine or nitroprusside — not a serotonergic event.
A tyramine crisis is a hypertensive emergency, not a serotonergic event. Normally, MAO-A in the gut wall and liver degrades dietary tyramine before it reaches systemic circulation. When MAO-A is inhibited, tyramine floods into the bloodstream, displaces norepinephrine from sympathetic nerve terminals, and causes a catecholamine surge — resulting in severe hypertension, headache, and risk of stroke. Treatment is alpha-blockade (phentolamine) or vasodilation (nitroprusside). Cyproheptadine (used for serotonin syndrome) would do nothing here.
Common mistake
Wrong: The washout period is the same length regardless of which drug (MAOI or SSRI) is being stopped.
Right: Stopping an MAOI before starting an SSRI requires a 2-week washout, but stopping fluoxetine (long half-life) before starting an MAOI requires a 5-week washout; other SSRIs require 2 weeks.
The washout rule is asymmetric and depends on which drug you're stopping, not just the drug class. When stopping an MAOI and starting an SSRI, wait 2 weeks — this allows MAO enzyme levels to recover. But when stopping fluoxetine specifically before starting an MAOI, you need 5 weeks, because fluoxetine has an active metabolite (norfluoxetine) with a half-life of 1–2 weeks, meaning the drug lingers far longer than other SSRIs. Stopping any other SSRI before an MAOI only requires 2 weeks. Getting this direction wrong on an exam question leads to the opposite answer.
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What the exam tests

  1. Know the difference between MAO-A and MAO-B isoforms: which substrates each degrades, which agents selectively inhibit each (selegiline = MAO-B at low doses), and which MAOIs are nonselective (phenelzine, tranylcypromine).
  2. Given a vignette of a patient on an MAOI who ate aged cheese or cured meats and now has a hypertensive emergency, identify this as a tyramine crisis — a sympathomimetic event — and select the correct antidote (phentolamine or nitroprusside, not serotonin antagonists).
  3. Calculate the correct washout period when switching between an MAOI and a serotonergic drug, and apply the asymmetric rule: fluoxetine requires 5 weeks before starting an MAOI, but stopping an MAOI only requires 2 weeks before starting any SSRI.

Can you avoid these mistakes?

A patient with Parkinson's disease is started on a low-dose MAO inhibitor. His neurologist says dietary tyramine restrictions are minimal. Which MAO isoform is being selectively inhibited, and which drug is most likely being used?
A 55-year-old on phenelzine for depression eats a plate of aged cheddar at a dinner party and develops a pounding headache and BP of 210/130. What is the mechanism of this crisis, and what drug do you reach for first?
You want to switch a patient from fluoxetine to phenelzine. The patient asks how long they need to wait between stopping one and starting the other. What is the correct washout period, and why is it different from switching other SSRIs to an MAOI?
A vignette describes a patient on an MAOI who is also taking meperidine for pain and develops agitation, hyperthermia, and clonus. Is this a tyramine crisis or serotonin syndrome, and how do you tell the difference?

Related topics

Selective Serotonin Reuptake Inhibitors (SSRIs) Major Depressive Disorder (MDD) Persistent Depressive Disorder (Dysthymia) Bipolar I Disorder Bipolar II Disorder

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