Trimethylaminuria (TMAU), also known as fish odor syndrome or stale fish syndrome, is a rare metabolic disorder characterized by persistent and socially distressing body odor resembling rotten or decaying fish due to the accumulation of trimethylamine in sweat, urine, and breath. Primary TMAU is caused by an autosomal recessive mutation resulting in deficiency or dysfunction of the hepatic enzyme flavin monooxygenase 3 (FMO3). Secondary TMAU may occur due to the accumulation of trimethylamine from excessive intake of dietary precursors such as choline in eggs, beans, and peas, and carnitine in red meats and fish. Additional causes of secondary TMAU may include liver failure, portosystemic shunting, menstruation, viral hepatitis, and testosterone therapy.

Clinically, there is no skin abnormality. TMAU may have significant psychosocial implications due to the strong body odor.

The incidence of TMAU is believed to be around 1 in 200 000 to 1 in 1 000 000. TMAU may affect both males and females. However, females may experience more severe symptoms with hormonal fluctuations. TMAU may also be more prevalent in Black individuals. TMAU may affect any age group, but it may be more noticeable upon puberty with hormonal changes. The odor may also fluctuate with changes in stress level, diet, and medications.