Short for monoamine oxidase inhibitor, a class of drugs that block the enzyme monoamine oxidase, which normally breaks down neurotransmitters such as serotonin, dopamine, and noradrenaline. Originally developed as antidepressants, MAO inhibitors remain in use for treatment-resistant depression and Parkinson’s disease. They carry a famous dietary restriction: because the enzyme also breaks down tyramine in foods, patients must avoid tyramine-rich items such as aged cheese, cured meats, fermented products, and certain beers, or risk a dangerous hypertensive crisis. This food-drug interaction makes dietary counselling an essential part of prescribing these medicines.