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.

Salsoline is a metabolite of dopamine and is highly prevalent in the urine and cerebrospinal fluid of alcoholics at the time of intoxication and for several days after. Salsoline and related compounds bind to the Type A Mono-amine Oxidase inducing neuronal cell death. At one time salsoline was thought to be a biomarker for Parkinson's disease. However, it was later shown that treatment regimes result in an increased endogenous concentration of salsoline and related compounds.

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 Iproniazid is no longer clinically prescribed and has been withdrawn due to incidences of hepatotoxicity.

Amezinium is a sympathomimetic used for its vasopressor effects in the treatment of hypotensive states. Amezinium inhibited monoamine oxidase (MAO) activity. Amezinium antagonized the response to tyramine and blocked neuronal uptake of noradrenaline. Side effects revealed are: palpitation, headache, nausea/vomiting, hot flashes, high blood pressure.