Pargyline, sold under the brand name Eutonyl among others, is a monoamine oxidase inhibitor (MAOI) medication which has been used to treat hypertension (high blood pressure) but is no longer marketed. It has also been studied as an antidepressant, but was never licensed for use in the treatment of depression. The drug is taken by mouth.

Side effects of pargyline include orthostatic hypotension among others. It has the potential for serious food and drug interactions with sympathomimetic agents like tyramine that can result in hypertensive crisis. Pargyline acts as a non-selective and irreversible inhibitor of the monoamine oxidases MAO-A and MAO-B. The exact mechanism of the hypotensive effects of pargyline and other MAOIs is unclear. Structurally, pargyline is a benzylamine derivative and is related to selegiline and clorgyline.

Pargyline was first described in 1960 and was introduced for medical use in 1963. It was available in the United States and the United Kingdom. The clinical use of pargyline was limited due to its side effects and interactions. The drug remained available in the United States as late as 2000. However, it was fully discontinued worldwide by 2007.

Pargyline is used as an antihypertensive agent in the treatment of hypertension (high blood pressure). The dosage was 12.5 to 200 mg per day. Its onset of action is slow and several weeks of continuous administration are required for the effects to develop fully upon initiation of treatment. The decrease in blood pressure with pargyline is described as impressive and is especially strong when standing. However, the blood pressure decrease with pargyline is often difficult to control adequately.

Pargyline shares its mechanism of action, monoamine oxidase inhibition, with a class of antidepressants that includes phenelzine, tranylcypromine, and isocarboxazid, among others. However, unlike other MAOIs, pargyline itself was never licensed for treatment of depression. In any case, the drug was studied in the treatment of depression and was advertised in the 1960s as an antihypertensive agent that also "brightens emotional outlook".

Orthostatic hypotension (excessively low blood pressure when standing or standing up) is a prominent side effect of pargyline. Other side effects include dry mouth, dizziness, nausea, headaches, increased appetite, nervousness, insomnia, agitation, sedation, manic reactions, and psychotic reactions.

Pargyline has the potential for serious food and drug interactions due to its MAOI actions. This includes hypertensive crisis with intake of norepinephrine releasing agents like tyramine, amphetamine, and ephedrine. Tyramine is found in high concentrations in certain cheeses and other foods and can result in hypertensive crisis often referred to as the "cheese reaction". Episodes of hypertensive crisis can be severe or fatal and this has greatly limited the clinical use of pargyline. Hypertensive crisis with pargyline is treated intravenously with sympatholytic alpha blockers like phentolamine.

Combination of pargyline and the antihypertensive agent methyldopa has been found to result in intense and potentially fatal central nervous system excitation in rodents. This has been said to resemble the effects of amphetamine overdose. The interaction appears to be due to inhibition by pargyline of the metabolism of normally short-lived methyldopa metabolites like α-methyldopamine and α-methylnorepinephrine that act as potent catecholamine releasing agents. Visual hallucinations have been reported with coadministration of pargyline and methyldopa in humans. As such, use of methyldopa in combination with pargyline and other MAOIs is contraindicated.