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MAO inhibitor (MAOI) n. a drug that prevents the activity of the enzyme monoamine oxidase (MAO) in brain tissue and therefore affects mood. The use of irreversible MAO inhibitors, such as phenelzine, as antidepressants is now restricted because their side-effects include interactions with other drugs and foods containing tyramine (e.g. cheese) to produce a sudden and dangerous increase in blood pressure. See also moclobemide, selegiline.