Cheese, Beer, and Serotonin: Making Sense of MAO Inhibitors

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Monoamine oxidase (or “MAO”) is an enzyme found in the human body. The MAO enzyme converts chemicals including dopamine, serotonin, and adrenaline into ammonia, hydrogen peroxide, and other products. There are two types of MAO found in the human body. MAO type A (or MAO-A) is found in the stomach and intestines, while most MAO type B (MAO-B) is located in other organs including the brain. 

Since depression is believed to be caused by low levels of “happy hormones” including serotonin, drugs that block or inhibit MAO have been used as antidepressants. MAO inhibitors keep the “happy hormones” from breaking down. This allows serotonin and other hormones to remain in the brain for longer, and helps to reduce symptoms of depression. These drugs are called “MAO inhibitors” and include phenelzine (Nardil®), tranylcypromine (Parnate®), and isocarboxazid (Marplan®). Because MAO is involved in the metabolism of multiple compounds in the human body and brain, MAO inhibitors have also been used to treat a variety of conditions including anxiety, dementia, and Parkinson’s disease. Since MAO inhibitors cause increases in serotonin and other natural chemicals, they can interact with other antidepressants or drugs that act in a similar way. Serotonin syndrome is a serious medical condition that is caused by the presence of excess serotonin in the human body. Serotonin syndrome can occur when an individual takes multiple medications or high doses of drugs that affect serotonin release or metabolism. Patients who develop serotonin syndrome often require specialized care or hospitalization. Because MAO inhibitors affect serotonin metabolism, people who take MAO inhibitors are at risk for developing serotonin syndrome.

One of the other chemicals affected by MAO inhibitors is tyramine. In the human body, the presence of excess tyramine causes high blood pressure along with other unpleasant symptoms such as nausea, skin flushing, and headache. This condition is often described as a “tyramine reaction”. Tyramine is present in the human digestive system and is normally metabolized there by MAO-A. The use of MAO inhibitor medications causes high levels of tyramine to build up in the body and can result in tyramine reactions. People who take higher doses of MAO inhibitors, as well as those who overdose on these medications, are at higher risk for developing tyramine reactions. Since tyramine is naturally present in some foods including aged cheeses and meats, draft beer, and sauerkraut, people who take standard prescribed doses of MAO inhibitors may develop tyramine reactions if they eat these foods. Because of this, individuals who take MAO inhibitors are often advised to follow a special low-tyramine diet.

Since tyramine is largely metabolized by MAO-A, the use of MAO-B inhibitors may reduce the risk of developing tyramine reactions. Newer MAO inhibitor drugs, including selegiline (Eldepryl®) and rasagiline (Azilect®), target MAO-B and are called “selective” MAO inhibitors. Selegiline is currently approved by the United States Food and Drug Administration as a treatment for adults with Parkinson’s disease. When used in low doses, selegiline is not associated with the development of tyramine reactions in most patients. However, when high doses of selegiline are taken or when the drug is taken in overdose, tyramine reactions can occur. Patients who experience tyramine reactions may require hospitalization, and the condition can be life-threatening in some cases. There is no antidote available for MAO inhibitor overdose or tyramine reactions, but there are medications available that can be used to reduce the unwanted and unpleasant signs and symptoms caused by these conditions.

Kelly Johnson-Arbor, MD
Medical Toxicologist