A monoamine oxidase inhibitor (MAOI) is a type of medication that works by blocking an enzyme in your brain responsible for breaking down certain mood-regulating chemicals. By preventing this breakdown, MAOIs allow serotonin, norepinephrine, and dopamine to build up to higher levels, which can relieve depression and other conditions. They were among the first antidepressants ever developed, and while newer drugs have largely replaced them as first-choice treatments, MAOIs remain uniquely effective for people whose depression hasn’t responded to other medications.
How MAOIs Work in the Brain
Your brain naturally produces an enzyme called monoamine oxidase, which acts like a cleanup crew. After neurotransmitters like serotonin, norepinephrine, and dopamine do their job sending signals between nerve cells, monoamine oxidase breaks them down so they don’t accumulate. In depression, these neurotransmitters are often in short supply or aren’t functioning effectively. MAOIs block the cleanup enzyme, letting those chemical messengers linger longer and continue their mood-stabilizing effects.
There are actually two forms of this enzyme. MAO-A primarily handles serotonin and norepinephrine, while MAO-B handles other brain chemicals like phenylethylamine. Dopamine is broken down by both forms. This distinction matters because drugs that target one form over the other have different uses and different risk profiles.
Types of MAOIs
MAOIs fall into a few categories depending on which enzyme form they target and whether their effects are permanent or temporary.
Non-Selective, Irreversible MAOIs
The oldest MAOIs block both MAO-A and MAO-B permanently. Once the enzyme is disabled, your body has to manufacture entirely new enzyme molecules before normal breakdown of neurotransmitters resumes, a process that takes about two weeks. Four of these are FDA-approved for depression: isocarboxazid, phenelzine, tranylcypromine, and selegiline (which at higher doses loses its selectivity). These are the most effective MAOIs for depression, but they also carry the most dietary restrictions.
Selective MAO-B Inhibitors
Drugs that target only MAO-B, like selegiline at low doses and rasagiline, are primarily used for Parkinson’s disease rather than depression. Because Parkinson’s involves a loss of dopamine-producing brain cells, boosting dopamine levels by blocking its breakdown can improve motor symptoms like tremor and stiffness. These selective inhibitors generally carry fewer dietary risks than the non-selective versions, since MAO-A in the gut and liver remains active and can still handle tyramine from food.
Reversible Inhibitors of MAO-A (RIMAs)
A newer class called RIMAs blocks only MAO-A, and does so temporarily. Unlike older MAOIs that permanently disable the enzyme, RIMAs can be displaced from it. This is a meaningful safety advantage: if you eat a food high in tyramine, the rising tyramine concentration can push the drug off the enzyme, allowing your body to process the tyramine normally. Meanwhile, MAO-B stays untouched and continues breaking down tyramine as well. The result is antidepressant activity with a much lower risk of dangerous blood pressure spikes. RIMAs are available in some countries but have limited availability in the United States.
What MAOIs Treat
Depression is the primary use, but MAOIs aren’t typically prescribed first. Current treatment guidelines position them as a third-line option or later, reserved for people who haven’t improved on other antidepressants. That said, they have a strong track record in specific situations.
Atypical depression responds particularly well to MAOIs. This subtype involves symptoms like increased sleep, increased appetite, a heavy or leaden feeling in the limbs, and intense sensitivity to rejection. Studies have shown MAOIs outperform tricyclic antidepressants for this pattern. MAOIs also show benefits for treatment-resistant depression (when multiple other medications have failed), depression with high anxiety, and certain forms of bipolar depression characterized by low energy. Some evidence supports their use in anxiety disorders and post-traumatic stress disorder as well.
For Parkinson’s disease, selective MAO-B inhibitors help manage early motor symptoms and can smooth out the fluctuations patients experience when taking levodopa, the standard Parkinson’s medication.
The Tyramine Problem
The most distinctive aspect of taking an MAOI is the need to avoid certain foods. This is sometimes called the “cheese effect” because aged cheese was the first food linked to dangerous reactions. The issue centers on tyramine, a substance found naturally in many foods. Normally, monoamine oxidase in your gut and liver breaks down tyramine before it can affect your blood pressure. When an MAOI blocks that enzyme, tyramine enters your bloodstream unchecked and can cause a sudden, severe spike in blood pressure.
This reaction, called a hypertensive crisis, typically hits one to two hours after eating the trigger food. Blood pressure can shoot up to ranges of 160/90 to 220/115 or higher. The main warning sign is a sudden, severe headache, often accompanied by a pounding heart and palpitations. In serious cases, the consequences can include stroke, bleeding in the brain, dangerous heart rhythms, and even death.
The foods you need to avoid share a common theme: they’re aged, fermented, cured, or spoiled. The major categories include:
- Aged and artisan cheeses: cheddar, Swiss, Parmesan, blue cheeses like Stilton and Gorgonzola, brie, Camembert, feta, Gruyere, and Edam
- Cured and processed meats: pepperoni, salami, dry sausage, bologna, bacon, corned beef, and smoked or cured fish
- Fermented foods: sauerkraut, kimchi, pickled fish, tofu, miso, and pickles
- Fermented sauces: soy sauce, fish sauce, shrimp sauce, Worcestershire sauce, and teriyaki sauce
- Fermented drinks: kombucha, kefir, and certain alcoholic beverages (especially tap beer, home-brewed beer and wine, and some red wines and sherries)
- Certain vegetables: fava beans and snow peas
Freshness matters broadly. Leftovers, overripe fruit, and anything past its prime can accumulate tyramine as bacteria break down proteins. Many clinicians recommend eating only fresh foods and being cautious with anything that’s been sitting in the refrigerator. Caffeinated beverages may also contain some tyramine and are often limited. Commercial pasteurized bottled beer and most commercial wines are generally considered safe in moderation.
Drug Interactions
MAOIs interact dangerously with a wide range of common medications, which is another reason they require careful management. The two main risks are serotonin syndrome and hypertensive crisis.
Combining an MAOI with any drug that raises serotonin levels can trigger serotonin syndrome, a potentially life-threatening condition involving agitation, high fever, rapid heart rate, and muscle rigidity. This rules out SSRIs (like fluoxetine and sertraline), SNRIs (like venlafaxine), and tricyclic antidepressants. Even dextromethorphan, the cough suppressant found in many over-the-counter cold medicines, is off limits. Certain antihistamines like chlorpheniramine and brompheniramine also increase serotonin and should be avoided.
On the blood pressure side, decongestants and other medications with stimulant-like effects are dangerous because they can amplify the same tyramine-related spike. Phenylephrine and oxymetazoline, found in many nasal sprays and cold remedies, fall into this category. Even epinephrine, used in some dental procedures and allergy treatments, poses a risk.
Because of these interactions, anyone on an MAOI needs to be cautious about medications that seem routine. Checking with a pharmacist before taking anything new, including supplements and over-the-counter products, is essential.
Common Side Effects
Beyond the food and drug restrictions, MAOIs carry a range of side effects that affect daily life. Dizziness when standing up is one of the most common, caused by a drop in blood pressure when you shift positions. Weight gain, insomnia, and sexual dysfunction also occur frequently. Some people experience dry mouth, drowsiness, or muscle twitching. These side effects, combined with the dietary burden, are the main reasons MAOIs have fallen behind newer antidepressants in everyday prescribing.
Stopping an MAOI also requires planning. Because the older versions permanently disable the enzyme, you generally need to wait at least two weeks after discontinuing before starting certain other antidepressants. This washout period allows your body to rebuild its monoamine oxidase supply. The same buffer applies in reverse: if you’re switching from an SSRI to an MAOI, you need to be off the SSRI long enough for it to fully clear your system.
Why MAOIs Still Matter
Despite the restrictions, MAOIs occupy an important niche. For people with severe, treatment-resistant depression who have tried multiple other medications without relief, MAOIs can be remarkably effective. Some psychiatrists argue they’re underused, noting that the dietary risks, while real, are manageable with proper education. The Canadian Network for Mood and Anxiety Treatments specifically recommends considering phenelzine for depression with atypical features, based on evidence that it outperforms other classes of antidepressants for that pattern. For the right patient, the extra vigilance around food and drug interactions is a reasonable trade-off for a medication that works when others haven’t.