An MAO inhibitor (MAOI) is a type of medication that works by blocking an enzyme called monoamine oxidase, which normally breaks down key mood-regulating chemicals in your brain. By disabling this enzyme, MAOIs allow serotonin, dopamine, and norepinephrine to build up, lifting mood and easing symptoms of depression. They were among the first antidepressants ever developed, and while newer drugs have largely replaced them as first-line treatments, MAOIs remain uniquely effective for certain types of depression that don’t respond to other medications.
How MAO Inhibitors Work
Your brain cells communicate using chemical messengers called neurotransmitters. Three of these, serotonin, dopamine, and norepinephrine, play central roles in regulating mood, motivation, energy, and emotional resilience. After these chemicals do their job, an enzyme called monoamine oxidase sits on the outer membrane of structures inside your cells and breaks them down, clearing them out so they don’t overstimulate the system.
In depression, the supply of these neurotransmitters can fall too low. MAOIs work by disabling the cleanup enzyme, which means more serotonin, dopamine, and norepinephrine accumulate in the gaps between brain cells. This increased concentration strengthens the chemical signals tied to mood and motivation. Most MAOIs bind to the enzyme permanently, destroying its function. Because the body needs roughly two weeks to manufacture fresh copies of the enzyme, the effects of these drugs linger well after you stop taking them.
Two Types of MAO Enzyme
The body produces two versions of monoamine oxidase, called MAO-A and MAO-B, and they handle slightly different jobs. MAO-A, found in the brain and throughout the gut, breaks down serotonin, norepinephrine, and dopamine. MAO-B, found in the brain and in blood platelets, primarily targets dopamine but leaves serotonin and norepinephrine alone.
This distinction matters for treatment. Drugs that block MAO-A tend to have the strongest antidepressant effects because they preserve all three mood-related neurotransmitters. Drugs that selectively block MAO-B are more useful in conditions like Parkinson’s disease, where the core problem is dopamine depletion. Some older MAOIs block both versions at once, which makes them potent but also increases the potential for side effects and food interactions.
There’s also a difference between irreversible and reversible inhibitors. Most MAOIs permanently disable the enzyme, meaning your body has to build new copies over about two weeks before normal enzyme activity returns. One exception is moclobemide, a reversible inhibitor that temporarily blocks the enzyme and releases it, which generally carries fewer dietary restrictions.
What MAOIs Treat
The FDA has approved three oral MAOIs for treating depression: isocarboxazid (Marplan), phenelzine (Nardil), and tranylcypromine (Parnate). A fourth, selegiline (Emsam), is available as a skin patch. These are rarely the first medication a doctor will try for depression, but they occupy an important role when other antidepressants haven’t worked.
MAOIs show a particular strength with a subtype called atypical depression, where people experience mood reactivity (your mood brightens temporarily in response to positive events), increased sleep, heavy feelings in the limbs, and sensitivity to rejection. Decades of research from Columbia University established that while many antidepressants work similarly for classic melancholic depression, patients with atypical depression respond preferentially to MAOIs. In one key trial of 90 patients with atypical depression, 83% responded to phenelzine compared with 50% on a tricyclic antidepressant and 19% on placebo. A larger trial of 119 patients found a 71% response rate for the MAOI versus 50% for the tricyclic and 28% for placebo.
Some evidence also supports MAOI use in early-stage treatment-resistant depression, particularly when patients have tried up to three other antidepressants without success. Results become less consistent in later-stage resistance, where multiple treatments have already failed.
The Tyramine Problem
The most well-known concern with MAOIs is a dangerous interaction with a substance called tyramine, found naturally in many foods. Normally, monoamine oxidase in your gut breaks down tyramine before it can affect your body. When an MAOI disables that enzyme, tyramine passes into your bloodstream unchecked, where it can trigger a rapid and potentially dangerous spike in blood pressure. This is sometimes called a “hypertensive crisis” and can require emergency treatment.
Tyramine builds up in foods that are aged, fermented, cured, or overripe. The highest-risk foods include:
- Aged cheeses like cheddar, Swiss, and blue cheese
- Cured and fermented meats like salami, pepperoni, and dried sausages
- Fermented soy products like soy sauce, miso, and tofu
- Yeast-extract spreads like Marmite and Vegemite
- Certain beans like fava beans (broad beans) and snow peas
- Overripe or dried fruits like raisins, overripe bananas, and overripe avocados
- Fermented or home-brewed alcohol, especially tap beer, home-brewed beer, and artisan wine
This dietary restriction is one of the main reasons MAOIs fell out of favor as newer antidepressants without food interactions became available. However, the selegiline patch offers a partial workaround. At its lowest dose (6 mg per 24 hours), the patch delivers the drug through the skin, largely bypassing the gut. At this dose, no dietary modifications are required. At higher doses (9 mg or 12 mg), tyramine restrictions apply, and they must continue for two weeks after lowering the dose or stopping the patch.
Drug Interactions
MAOIs interact dangerously with several other medications, and this is just as important as the food restrictions. Combining an MAOI with drugs that also raise serotonin levels can cause serotonin syndrome, a potentially life-threatening condition marked by agitation, high fever, muscle rigidity, and rapid heart rate.
The most critical interactions involve other antidepressants, particularly SSRIs (like fluoxetine or sertraline) and SNRIs (like venlafaxine or duloxetine). These drug classes should never be taken alongside an MAOI, and because most MAOIs irreversibly disable the enzyme, a washout period of at least two weeks is typically required when switching between an MAOI and another antidepressant. Certain pain medications, particularly those related to opioids, and some over-the-counter cold and cough medications containing dextromethorphan, also pose serious risks.
This web of interactions means that anyone taking an MAOI needs to check with a pharmacist before starting any new medication, including supplements and over-the-counter drugs.
Common Side Effects
Beyond the food and drug interactions, MAOIs carry a side effect profile that overlaps with other antidepressants but has a few distinctive features. Dizziness when standing up (orthostatic hypotension) is one of the more common complaints, caused by a drop in blood pressure when you shift from sitting or lying down to standing. This can be especially noticeable in the first few weeks of treatment.
Other frequently reported effects include insomnia or disrupted sleep, dry mouth, weight gain, and sexual side effects like difficulty with arousal or orgasm. Some people experience muscle twitching or a tingling sensation in the skin. These side effects vary considerably from person to person and between different MAOIs, so switching to a different one within the class sometimes helps.
Why MAOIs Still Matter
Despite being older medications with dietary and drug restrictions that make them less convenient, MAOIs fill a gap that newer antidepressants often can’t. Their effectiveness in atypical depression is well-documented and, in some studies, unmatched. For patients who have cycled through SSRIs, SNRIs, and other modern antidepressants without adequate relief, an MAOI can offer a meaningfully different mechanism of action, one that raises dopamine in addition to serotonin and norepinephrine.
The development of the selegiline patch has also made the class more accessible by removing dietary restrictions at the lowest effective dose. While MAOIs will likely never return to widespread first-line use, they remain one of the most potent tools available for depression that resists other treatments.