Clinical Psychopharmacology · Paper A
MAOI dietary interactions (tyramine)
MAOIs (monoamine oxidase inhibitors, an older class of antidepressant) such as phenelzine and tranylcypromine block the gut and liver enzyme that normally breaks down tyramine, a substance found in many aged and fermented foods. With that enzyme blocked, dietary tyramine is absorbed instead of destroyed and floods the body. A patient on phenelzine who eats a meal of mature cheddar and cured salami can trigger a medical emergency within the hour from that single meal. Every patient started on an irreversible MAOIMonoamine oxidase inhibitor, an older antidepressant with strict food and drug interaction rules.Read more → therefore needs an explicit, written list of foods to avoid, not just a verbal warning.
The absorbed tyramine enters sympathetic nerve endings and pushes out stored noradrenaline (the 'fight-or-flight' transmitter that tightens blood vessels). An hour after that cheese-and-salami meal, the same patient develops a pounding headache at the back of the head, palpitations, neck stiffness, sweating and chest pain: the hypertensive crisis in progress. The patient must avoid high-tyramine foods: mature cheeses, cured or fermented meats, yeast extracts, fermented soya, and some unpasteurised draught beers. Fresh food and ordinary pasteurised dairy remain low risk and do not need restricting. Take a patient on tranylcypromine who attends a party and eats mature cheddar with a glass of tap beer, e.g. within the hour he develops a severe throbbing occipital headache and a dangerously high blood pressure reading. Clinically, any MAOIMonoamine oxidase inhibitor, an older antidepressant with strict food and drug interaction rules.Read more → patient presenting with a sudden severe headache after eating out should be assessed for a hypertensive crisis first, before other causes of headache are considered.
Very few psychiatric drug interactions can kill a patient within an hour of a single meal. The -tyramine reaction is one of them, which is why it remains a fixture of the exam despite MAOIs (monoamine oxidase inhibitors) being prescribed far less often than they once were: the stakes of missing it are simply too high to drop from the syllabus.
An (monoamine oxidase inhibitor) is an older class of antidepressant that blocks the enzyme monoamine oxidase, which normally breaks down monoamine and dietary amines in the gut and liver. TYRAMINE (an amino-acid-derived compound found in aged, fermented and cured foods) is normally destroyed by this enzyme before it reaches the circulation in any meaningful amount. A PRESSOR reaction is a sudden, dangerous rise in blood pressure, distinct from the separate danger of , which involves excess serotonin activity rather than a noradrenaline surge.
A common trap is assuming any carries the same dietary risk. Suppose a stem describes a patient on moclobemide, a reversible -A-selective inhibitor, who eats a modest amount of mature cheese: the risk here is much lower than with an irreversible agent like phenelzine or tranylcypromine, because moclobemide's inhibition is displaced by the surge of tyramine itself, limiting the reaction.
Start from the enzyme. Monoamine oxidase in the gut wall and liver ordinarily destroys tyramine before it is absorbed: this is called the first-pass effect. Block that enzyme irreversibly, and tyramine from food is absorbed intact instead. Once in the circulation, tyramine enters sympathetic nerve terminals and displaces stored noradrenaline (the behind the body's fight-or-flight response), flooding the bloodstream with it and driving blood pressure sharply upward.
A patient on an irreversible such as phenelzine eats a meal containing mature cheddar and cured salami, both high in tyramine. Within roughly an hour, the tyramine load overwhelms the residual, non-drug-blocked enzyme elsewhere in the body, is absorbed, and triggers a surge of noradrenaline release from nerve terminals. The patient develops a pounding occipital (back-of-head) headache, palpitations, neck stiffness, sweating, and chest pain: the hypertensive crisis in progress, which can proceed to intracranial haemorrhage if untreated.
Do not confuse the tyramine pressor reaction with , which instead follows combining an with a drug (for example another antidepressant), producing a different picture of agitation, high fever, muscle rigidity and (rhythmic muscle jerking) rather than the headache-dominant, blood-pressure-driven picture of a tyramine reaction. Both are MAOI-related emergencies, but they need to be told apart because their typical trigger and clinical picture differ.
Diagnosis is largely clinical: a patient on an who develops a sudden severe headache with hypertension after a meal should be treated as a pressor crisis until proven otherwise. Blood pressure measurement confirms the severity; there is no specific blood test that confirms tyramine as the trigger, so the history of recent food intake is the key diagnostic clue.
Management of an established crisis is an emergency: urgent blood pressure control, typically with a rapidly acting antihypertensive, and hospital assessment for end-organ damage. Prevention is the real management strategy: every patient started on an irreversible needs an explicit, written list of high-tyramine foods to avoid (mature cheeses, cured or fermented meats, yeast extracts, fermented soya products, and some unpasteurised draught beers), not just a verbal warning, following Maudsley Prescribing Guidelines and BNF advice on MAOI dietary restriction. Fresh food and ordinary pasteurised dairy remain low risk. Because the enzyme block from an irreversible MAOI is permanent until new enzyme is made, dietary restriction must continue for about two weeks after stopping the drug.
A stem combining an with cheese, cured meat, or a fermented product is testing the tyramine pressor reaction. A stem combining an MAOI with another antidepressant or drug is testing instead. A patient who stops phenelzine and, ten days later, eats a tyramine-rich meal is still at risk: the two-week washout window has not yet closed.
MAOIs block the enzyme that normally destroys dietary tyramine, so tyramine-rich foods in a patient on an irreversible are absorbed intact, displace stored noradrenaline, and can trigger a hypertensive crisis within about an hour, distinct from the separate serotonin-syndrome risk of combining an MAOI with another drug, and this is why every patient starting an irreversible MAOI needs an explicit, written dietary restriction list that continues for roughly two weeks after the drug is stopped.