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.

Clorgiline is a monoamine oxidase (MAO) inhibitor. Specifically, it is an irreversible and selective inhibitor of MAO-A. Clorgiline was under investigation for antidepressant and anxiolytic potential but has never been marketed, likely due to efficacy concerns. It continues to see routine use as a molecular probe in biomedical research examining a number of neurological disease and cancer models. In addition to inhibiting the MAO-A receptor, it has also been found to bind to the sigma1 receptor, and with high affinity to the I2 imidazoline receptor.

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.

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 It was previously used as an antidepressant (trade name Niamid) but was withdrawn by Pfizer in 1963 due to the risk of hepatotoxicity.

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 Iproniazid is no longer clinically prescribed and has been withdrawn due to incidences of hepatotoxicity.

Eprobemide (brand name Befol/Бефол), a drug used in Russia, is a selective, reversible monoamine oxidase A (MAO) inhibitor, which causes deamination of noradrenaline, serotonin, partially dopamine and β-phenylethylamine, thus increases the content of neurotransmitter monoamines in the central nervous system. Befol does not affect the deaminination of the tyramine and does not cause its accumulation in the body, in contrast to non-selective inhibitors of irreversible action, that is why the drug causes fewer side effects. Befol is used in adults with depression of different nature: endogenous depression in manic-depressive psychosis and various forms of schizophrenia, depression in patients with chronic alcoholism.

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.

Caroxazone, 2-oxo-2H-1,3-benzoxazine-3(4H)-acetamide, was a psychotropic agent which in clinical studies was shown to possess a relevant antidepressant activity and was used to treat depression. This drug acts as a reversible inhibitor on both monoamine oxidases, MAO-A and MAO-B. Caroxazone is no longer marketed.

Indoles and their derivatives are well-known as an important class of heterocyclic compounds, their core being a near-ubiquitous component of biologically active natural products, widespread in different species of plants, animals, and marine organisms. The indole is also well-known as one of the most important scaffolds for drug discovery, capable of serving as ligand for a diverse array of receptors. Indoles is used in textile dyes, perfumes, in agriculture and veterinary medicine. Relatively new areas are dietary supplements and nutraceuticals.

Clorgiline is a monoamine oxidase (MAO) inhibitor. Specifically, it is an irreversible and selective inhibitor of MAO-A. Clorgiline was under investigation for antidepressant and anxiolytic potential but has never been marketed, likely due to efficacy concerns. It continues to see routine use as a molecular probe in biomedical research examining a number of neurological disease and cancer models. In addition to inhibiting the MAO-A receptor, it has also been found to bind to the sigma1 receptor, and with high affinity to the I2 imidazoline receptor.