An MAOI, or monoamine oxidase inhibitor, is a class of medication that treats depression and Parkinson’s disease by blocking an enzyme in your brain that breaks down mood-regulating chemicals. MAOIs were among the first antidepressants ever discovered, and while newer drugs have largely replaced them as a first choice, they remain uniquely effective for people whose depression hasn’t responded to other treatments.
How MAOIs Work in the Brain
Your brain produces chemical messengers called neurotransmitters, including serotonin, norepinephrine, and dopamine, that regulate mood, motivation, and alertness. After these chemicals do their job, an enzyme called monoamine oxidase breaks them down and clears them away. In depression, the supply of these neurotransmitters can run too low.
MAOIs block that cleanup enzyme, allowing serotonin, norepinephrine, and dopamine to build up and remain active longer. More of these chemicals available in the brain means stronger signaling between nerve cells, which over time lifts mood and reduces anxiety. The effect isn’t instant. Like most antidepressants, MAOIs typically take several weeks to produce their full benefit.
Two Types of MAO Enzymes
There are actually two forms of the monoamine oxidase enzyme, called MAO-A and MAO-B, and they have different roles. MAO-A has a stronger preference for breaking down serotonin and norepinephrine, the chemicals most closely tied to mood. MAO-B prefers other compounds, though both forms break down dopamine roughly equally.
This distinction matters because it splits MAOIs into two practical categories. Drugs that block MAO-A raise serotonin and norepinephrine levels in the brain and work as antidepressants. Drugs that selectively block MAO-B raise dopamine levels and are used to treat Parkinson’s disease, where dopamine-producing brain cells gradually die off. Some older MAOIs block both enzyme types at once.
What MAOIs Are Prescribed For
MAOIs are not first-line treatments. They’re typically reserved for people who haven’t improved on more commonly prescribed antidepressants. Canadian clinical guidelines, for example, list phenelzine as a third-line option for depression, with a specific recommendation for depression with “atypical features,” a subtype characterized by oversleeping, overeating, heavy feelings in the limbs, and strong sensitivity to rejection. In head-to-head comparisons, MAOIs have shown superiority over older tricyclic antidepressants for this particular pattern of depression.
The FDA-approved MAOIs currently available in the United States include:
- Phenelzine: approved for atypical and treatment-resistant depression
- Tranylcypromine: approved for treatment-resistant major depression
- Isocarboxazid: approved for major depression that hasn’t responded to other treatments
- Selegiline: available as a pill for Parkinson’s disease and as a skin patch for depression
- Rasagiline and safinamide: approved for Parkinson’s disease
MAOIs for Parkinson’s Disease
The MAO-B inhibitors selegiline and rasagiline play a different role in Parkinson’s disease. By blocking the enzyme that breaks down dopamine, they help preserve the dwindling dopamine supply in a brain that’s progressively losing its dopamine-producing cells. They’re often used alongside levodopa, the primary Parkinson’s medication, and clinical trials have shown rasagiline can reduce the required levodopa dose.
These drugs also appear to do more than just manage symptoms. Research shows they protect brain cells from the kind of damage that drives Parkinson’s progression. They reduce oxidative stress, support healthy function in mitochondria (the energy-producing structures inside cells), and boost the brain’s production of protective growth factors. In one landmark trial called ADAGIO, patients who started rasagiline earlier showed better outcomes than those who began the same drug later, suggesting a genuine slowing of disease progression rather than just symptom relief.
The Tyramine Problem
The most well-known drawback of MAOIs is a strict set of dietary restrictions, sometimes called the “cheese effect.” Here’s why: tyramine is a natural compound found in many fermented and aged foods. Normally, the MAO enzyme in your gut breaks down tyramine before it reaches your bloodstream. When you’re taking an MAOI, that safety mechanism is partially or fully shut off. Tyramine passes through intact, and high levels of it in your blood can cause a sudden, dangerous spike in blood pressure that requires emergency treatment.
The list of foods to avoid is extensive:
- Aged and artisan cheeses: cheddar, Swiss, Parmesan, blue cheeses, brie, Camembert, feta, Gruyere
- Cured and processed meats: salami, pepperoni, dry sausage, bologna, bacon, corned beef, smoked fish
- Fermented foods: sauerkraut, kimchi, pickles, tofu, miso, soy sauce, fish sauce, Worcestershire sauce
- Fermented drinks: kombucha, kefir, craft or home-brewed beer, some red wines, sherry, liqueurs
- Other high-tyramine foods: fava beans, dried or overripe fruits, yeast-extract spreads like Marmite and Vegemite, meat tenderizers
- Improperly stored food: anything spoiled or left out too long
There is one partial exception. The selegiline transdermal patch, which delivers medication through the skin, bypasses the digestive system entirely. At its lowest dose (6 mg per 24 hours), the patch doesn’t significantly affect the MAO enzyme in the gut, so dietary restrictions aren’t required. At higher doses (9 or 12 mg), the patch does begin to inhibit intestinal MAO, and the food restrictions apply again.
Dangerous Drug Interactions
MAOIs interact dangerously with two broad categories of drugs: those that raise blood pressure through stimulant-like effects, and those that increase serotonin levels. These interactions can be life-threatening, and some involve medications you might not think twice about taking.
The most critical interaction is with SSRIs and SNRIs, the antidepressants most commonly prescribed today. Combining either class with an MAOI floods the brain with serotonin and can trigger serotonin syndrome, a potentially fatal condition marked by confusion, agitation, rapid heart rate, high blood pressure, muscle rigidity, and seizures. This combination has caused deaths even at normal therapeutic doses. If you’re switching from an SSRI or SNRI to an MAOI, a washout period of at least two weeks is necessary to let the first drug clear your system. For fluoxetine (Prozac), which lingers in the body much longer, the required gap is five weeks.
Common over-the-counter cold and cough medicines are also a serious concern. Decongestants containing phenylephrine, pseudoephedrine, or oxymetazoline can add to the blood-pressure-raising effects of MAOIs. Dextromethorphan, the cough suppressant found in many cold remedies, is a weak serotonin reuptake inhibitor and poses a risk of serotonin syndrome. Certain pain medications, including tramadol and meperidine, carry the same serotonin-related risk. People taking MAOIs need to check the active ingredients of any medication, even ones sold without a prescription.
Why MAOIs Are Still Prescribed
Given the dietary restrictions and interaction risks, it’s fair to wonder why anyone would take an MAOI. The answer is simple: for some people, nothing else works. Treatment-resistant depression affects a significant portion of people with major depression, and after multiple failed trials of SSRIs, SNRIs, and other modern antidepressants, MAOIs can produce real improvement where other drugs could not.
MAOIs were actually discovered by accident. In 1951, doctors testing a drug called iproniazid as a tuberculosis treatment noticed that patients became unexpectedly euphoric. That observation eventually led to the development of the entire MAOI class. While the restrictions they require have pushed them to the margins of psychiatry, they remain a powerful tool. The risks are manageable with proper education about food and drug interactions, and for patients who have exhausted other options, the benefit of finally finding an effective antidepressant often outweighs the burden of a modified diet.