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FMO3 gene
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Gene

FMO3 gene

The FMO3 gene provides instructions for making an enzyme that breaks down nitrogen-containing compounds from the diet and plays a role in processing certain medications.

Normal Function

The FMO3 gene provides instructions for making an enzyme that is part of a larger enzyme family called flavin-containing dimethylaniline monooxygenases (FMOs). These enzymes break down compounds that contain nitrogen, sulfur, or phosphorus. The FMO3 enzyme, which is made chiefly in the liver, is responsible for breaking down nitrogen-containing compounds derived from the diet.

One of these compounds is trimethylamine, which is the molecule that gives fish their fishy smell. Trimethylamine is produced as bacteria in the intestine help digest certain proteins obtained from eggs, liver, legumes (such as soybeans and peas), certain kinds of fish, and other foods. The FMO3 enzyme normally converts fishy-smelling trimethylamine into another compound, trimethylamine-N-oxide, which has no odor. Trimethylamine-N-oxide is then excreted from the body in urine.

Researchers believe that the FMO3 enzyme also plays a role in processing some types of drugs, including:

  • The anticancer drug tamoxifen
  • The anti-inflammatory medication benzydamine
  • The antifungal drug ketoconazole
  • Certain medications used to treat depression (antidepressants)

The FMO3 enzyme may also be involved in processing nicotine. Normal variations (polymorphisms) in the FMO3 gene may affect the enzyme's ability to break down these substances.

Health Conditions Related to Genetic Changes

Trimethylaminuria

More than 40 variants (mutations) in the FMO3 gene have been identified in people with trimethylaminuria. Some of these variants lead to the production of a small, nonfunctional version of the FMO3 enzyme, while others change single amino acids, altering the enzyme's shape and disrupting its function.

Without enough functional FMO3 enzyme, the body is unable to convert trimethylamine into trimethylamine-N-oxide effectively. As a result, trimethylamine builds up in the body and is released in an affected person's sweat, urine, and breath, causing a strong fish-like body odor. Studies suggest that diet and stress also play a role in determining the intensity of the odor.

Genomic Location

The FMO3 gene is found on chromosome 1.

Frequently Asked Questions

Individuals with trimethylaminuria should strictly limit or avoid foods high in trimethylamine or its precursor choline. Key dietary restrictions include most fish (especially oily varieties like mackerel, sardines, and tuna), shellfish, liver, egg yolks, and certain legumes such as soybeans and peas. Additionally, some individuals may need to avoid certain vegetables like broccoli and cauliflower, as well as specific supplements containing choline, creatine, or carnitine, as these can exacerbate the fishy odor.
Trimethylaminuria is a lifelong genetic condition, but it is not progressive in the sense that the underlying enzyme deficiency does not worsen over time. However, the intensity of the odor can fluctuate based on diet, stress, and hormonal changes. While dietary modification is the primary management strategy, it is not the only option. Some patients find relief through the use of activated charcoal or specific probiotics that help reduce intestinal production of trimethylamine. In severe cases, clinicians may consider other supportive measures, but there is currently no cure to restore full enzyme function.
Trimethylaminuria is a specific metabolic disorder caused by FMO3 gene mutations that prevent the conversion of trimethylamine into odorless trimethylamine-N-oxide. Other causes of fishy body odor include bacterial vaginosis, certain skin infections, or the consumption of large amounts of fish without a metabolic disorder. A definitive diagnosis of trimethylaminuria requires genetic testing to identify FMO3 variants or a biochemical test showing elevated levels of trimethylamine in the urine, distinguishing it from transient or infectious causes of odor.
The FMO3 enzyme is involved in the breakdown of several drugs, including tamoxifen, benzydamine, ketoconazole, and certain antidepressants. Individuals with reduced FMO3 activity may metabolize these medications more slowly, potentially leading to higher blood levels of the drug and an increased risk of side effects. Conversely, some variants may alter the efficacy of the medication. Patients with known FMO3 variants should inform their healthcare providers, as dosage adjustments or alternative medications may be necessary to ensure safe and effective treatment.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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