MAOIs (monoamine oxidase inhibitors) are a class of medications that increase levels of key mood-regulating chemicals in your brain by blocking the enzyme that normally breaks them down. They were among the first antidepressants ever developed and remain effective options for depression, particularly cases that haven’t responded to newer medications. They’re also used to manage movement symptoms in Parkinson’s disease.
How MAOIs Work in the Brain
Your brain relies on chemical messengers called neurotransmitters to regulate mood, motivation, alertness, and movement. Three of the most important are serotonin, norepinephrine, and dopamine. After these chemicals do their job, an enzyme called monoamine oxidase breaks them down so they don’t accumulate. This cleanup process happens continuously on the outer membrane of mitochondria, the energy-producing structures inside your cells.
MAOIs block this cleanup enzyme, which means serotonin, norepinephrine, and dopamine stick around longer and build up to higher concentrations. The result is stronger signaling between nerve cells. In depression, where these neurotransmitter levels are often too low, this boost can relieve symptoms like persistent sadness, low energy, and difficulty concentrating.
There are actually two versions of the enzyme. MAO-A primarily breaks down serotonin, norepinephrine, and epinephrine. MAO-B primarily breaks down dopamine and a stimulant-like compound called phenethylamine. Some MAOIs block both versions, while others target just one. This distinction matters because it determines what a given MAOI is best suited to treat.
One important detail: older MAOIs block the enzyme permanently. They don’t just temporarily slow it down. Once the enzyme is disabled, your body has to manufacture entirely new copies, a process that takes several weeks. This is why the effects of these medications (and their dietary restrictions) linger well after you stop taking them.
Conditions MAOIs Treat
The FDA has approved three oral MAOIs for depression: isocarboxazid (Marplan), phenelzine (Nardil), and tranylcypromine (Parnate). A fourth, selegiline (Emsam), is available as a skin patch. These medications are typically reserved for people whose depression hasn’t improved with more commonly prescribed antidepressants like SSRIs or SNRIs, though some clinicians consider them earlier for specific depression subtypes.
MAO-B selective inhibitors play a different role in Parkinson’s disease. Since MAO-B is the version of the enzyme that primarily breaks down dopamine, blocking it helps preserve the dopamine that Parkinson’s patients are already losing due to their disease. The Parkinson’s Foundation notes these drugs provide modest benefits for movement symptoms and can be used alone in early stages or alongside other Parkinson’s medications. When combined with levodopa (the main Parkinson’s drug), MAO-B inhibitors help reduce “off” periods, those stretches when medication wears off and symptoms return, while extending the time patients feel their medication is working.
Common Side Effects
The most frequently reported side effect when starting an MAOI is orthostatic hypotension, a drop in blood pressure when you stand up that can cause dizziness or lightheadedness. Other early side effects include drowsiness, insomnia, and nausea. These tend to be most noticeable in the first few weeks.
With longer-term use, the side effect profile shifts. Fluid retention, muscle pains, weight gain, and sexual dysfunction become more common. Some people experience paresthesias, an abnormal tingling or prickling sensation in the skin. Involuntary muscle twitches can also occur. Among the oral MAOIs, phenelzine tends to cause more sedation and weight gain than the others.
The Tyramine Problem
The most distinctive risk of MAOIs involves food. MAO-A doesn’t just break down neurotransmitters in the brain. It also breaks down tyramine, a compound found naturally in many aged and fermented foods. When MAO-A is blocked by medication, tyramine from your diet can build up rapidly in the body, triggering a dangerous spike in blood pressure that may require emergency treatment.
Foods high in tyramine that need to be avoided include aged cheeses, fermented meats (like salami and pepperoni), fava beans, soy sauce, marmite, and certain beers. This dietary restriction is sometimes called the “cheese effect” because aged cheese was one of the first foods linked to these blood pressure crises.
The selegiline skin patch offers a notable workaround. Because the medication is absorbed through the skin rather than passing through the gut and liver, it avoids exposing the digestive system’s MAO-A to the drug. In clinical trials involving over 2,500 patients, no hypertensive crises were reported, even without dietary restrictions. The FDA ultimately approved the lowest-dose patch without requiring any food restrictions at all, though higher-dose patches still carry dietary guidelines. Tyramine challenge studies showed that the low-dose patch had a safety margin roughly ten times greater than what you’d encounter in even a tyramine-rich meal.
Drug Interactions
MAOIs interact dangerously with a wide range of other medications. The most serious risk is serotonin syndrome, a potentially life-threatening condition caused by too much serotonin activity in the brain. Symptoms include agitation, rapid heartbeat, high body temperature, and muscle rigidity. This can happen when MAOIs are combined with other drugs that also raise serotonin levels, including SSRIs (like fluoxetine or sertraline), SNRIs (like venlafaxine), certain pain medications, and some over-the-counter cough and cold products.
Because older MAOIs permanently disable the enzyme, a washout period of several weeks is typically necessary before starting or switching to another antidepressant. This waiting period gives the body time to produce fresh monoamine oxidase. The same applies in reverse: if you’re switching from an SSRI to an MAOI, the SSRI needs to fully clear your system first.
Why MAOIs Are Still Used
Despite their dietary restrictions and interaction risks, MAOIs remain in use because they work through a fundamentally different mechanism than newer antidepressants. SSRIs and SNRIs prevent neurotransmitters from being reabsorbed after release. MAOIs prevent them from being broken down entirely. This means they affect neurotransmitter levels in a broader, more sustained way, boosting serotonin, norepinephrine, and dopamine simultaneously rather than targeting just one or two.
For people with treatment-resistant depression or certain subtypes like atypical depression (characterized by increased appetite, excessive sleep, and sensitivity to rejection), MAOIs can succeed where other antidepressants have failed. The trade-off is managing the dietary restrictions and being vigilant about drug interactions, which is why they’re typically prescribed by psychiatrists with experience using them.