Monoamine oxidase inhibitors (MAOI) in clinical use have an irreversible action on MAO, and this persists until the enzyme has been resynthesized. The effects of small daily doses of MAOI are therefore cumulative. The biochemical effects of these drugs will involve several substrates of MAO, e.g. dopamine, tyramine, serotonin and, to a lesser extent, noradrenaline and adrenaline.