Pharmacology · Class
Monoamine oxidase inhibitors (MAOIs) !
Antidepressants that block the enzyme breaking down serotonin, norepinephrine and dopamine. Reserved for resistant cases because of food and drug interactions.
Updated 2026-11-01
What they are
Antidepressants that inhibit monoamine oxidase, the enzyme that degrades monoamines inside neurons and in the gut and liver. They are effective but reserved for depression that has not responded to other drugs, because of dietary restrictions (tyramine) and dangerous interactions.
Mechanism of action
Monoamine oxidase (MAO-A and MAO-B, on the outer mitochondrial membrane) oxidises serotonin, norepinephrine, dopamine and dietary amines such as tyramine. Phenelzine and tranylcypromine inhibit both forms; tranylcypromine does so irreversibly, so activity only returns as new enzyme is made. More monoamine is stored and released by neurons; in the gut and liver, tyramine from food is no longer destroyed.
Target: Monoamine oxidase A and B (MAOA, MAOB)
Drugs in this class
Others in the class
- Tranylcypromine
- Isocarboxazid
- Moclobemide (reversible MAO-A inhibitor)
- Selegiline (MAO-B, used in Parkinson disease)
Test yourself
Think of the answer, then tap the question to see it.
Why can a slice of aged cheese cause a hypertensive crisis in a patient on an MAOI?
Tyramine in the cheese is normally destroyed by MAO-A in the gut wall and liver. With MAO blocked, tyramine reaches the circulation and releases stored norepinephrine from sympathetic nerve endings, sharply raising blood pressure.
Why does switching from an MAOI to an SSRI need a waiting period?
MAO stays inhibited for some time after the last dose (irreversibly with tranylcypromine, until new enzyme is made), so starting an SSRI too soon can cause serotonin syndrome. The label sets the interval.
Which opioid and which cough suppressant are classic dangerous combinations with MAOIs?
Meperidine (pethidine) and dextromethorphan, which can cause a serotonin-syndrome-like reaction with agitation, hyperthermia, seizures and coma.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - PHENELZINE SULFATE tablet, film coated (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - TRANYLCYPROMINE SULFATE tablet, film coated (DailyMed (U.S. National Library of Medicine), 2025); Phenelzine: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2017); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).
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Representative model, for educational purposes only.