These are powerful, quick acting preparations that destroy or prevent the action of monoamine oxidase. Monoamines play an important part in the chemistry of the brain, and it is thought that an accumulation of them broken down by the enzyme, monoamine oxidase can excite the brain, leading to symptoms such as high blood pressure, sweating, palpitations and a feeling of suffocation. Therefore, these drugs prevent the enzyme from breaking down the monoamines.

These substances are valuable in the treatment of depression with anxiety. The dosage is usually one tablet in the morning and one in the afternoon. If taken too late in the afternoon, they may make sleep less deep. They work quite quickly and should, if possible, be used for only a limited time. They should not be discontinued abruptly but should be gently tailed off.

They must be used with great care and should be avoided by people with heart conditions, with conditions were the blood vessels in the brain are involved, and with epilepsy. Certain foods must be avoided, such as meat and yeast extracts (such as Marmite), cheese, alcohol, pickled herrings, tinned fish and textured vegetable protein foods. This because these foods contain certain substances which increase the amount of monoamines in body. No other drugs should be taken at the same time without medical advice, and it is customary now or pharmacists to supply a card with detailed instructions when giving out monoamine oxidase inhibitors.