Harmine (aka telepathine) is a fluorescent harmala alkaloid belonging to the beta-carboline family of compounds. It is a naturally occurring metabolite in a number of plants, notably the Middle Eastern plant harmal or Syrian rue (Peganum harmala) and the South American vine Banisteriopsis caapi. Harmine is a reversible inhibitor of monoamine oxidase A (MAO-A), but not MAO-B. Harmine has been found to have potential anti-cancer and neuroprotective properties, and also promotes differentiation of osteoblasts and chondrocytes while inhibiting osteoclastogenesis. Harmine has also been used as a C-11 labeled probe in positron emission tomography.

Pargyline is an irreversible selective monoamine oxidase (MAO)-B inhibitor, which possesses higher selectivity to this isoform in comparison with MAO-A. It was approved under brand name eutonyl for the treatment hypertension, but then this drug was discontinued.

ETRYPTAMINE (MONASE®), similar to the hallucinogenic tryptamines, is an inhibitor of monoamine oxidase, introduced for use as an antidepressant. It was withdrawn from the market due to problems with agranulocytosis and other side effects. However, it's activity is still under scientific investigation.

Nialamide is a non-selective, irreversible monoamine oxidase inhibitor of the hydrazine class. It was previously used as an antidepressant (trade name Niamid) but was withdrawn by Pfizer in 1963 due to the risk of hepatotoxicity.

PHENIPRAZINE is a monoamine oxidase inhibitor of the hydrazine chemical class that was used for the treatment of depression, schizophrenia, and also as a long-acting coronary vasodilator in patients suffering from angina pectoris. PHENIPRAZINE was discontinued due to toxicity concerns such as jaundice, amblyopia, and optic neuritis.

Iproniazid is a non-selective, irreversible monoamine oxidase inhibitor (MAO) of the hydrazine class. It was originally developed for the treatment of Tuberculosis, but in 1952, its antidepressant properties were discovered when researchers noted that patients given isoniazid became inappropriately happy. Iproniazid is no longer clinically prescribed and has been withdrawn due to incidences of hepatotoxicity.

Iproclozide [1-(p-chlorophenoxyacetyl)-2-isopropylhydrazine] is a non-selective monoamine oxidase inhibitor. It has been used in the treatment of depressive states. Iproclozide was studied in the treatment of hypertension and angina pectoris. It may induce fatal hepatitis.

Lazabemide is a reversible and selective inhibitor of monoamine oxidase B (MAO-B) that was under clinical development against Parkinson's disease, Alzheimer's disease and as an aid to smoking cessation. The development of the drug was discontinued due to liver toxicity.

Safrazine (Safra) is a non-selective, irreversible monoamine oxidase inhibitor (MAOI) of the hydrazine class that was introduced as an antidepressant in the 1960s. Safrazine and electroconvulsive therapy were shown to be effective in the treatment of antidepressant-resistant depressions (ARD) but had the drawbacks of the high incidence of moderate or severe adverse reactions and high relapse rates, respectively. The use of safrazine was discontinued.