Why does my breath smell like fish?

The classic fish smell is trimethylamine. MedlinePlus Genetics describes it as smelling like rotten or decaying fish. Gut bacteria make it when you digest foods such as eggs, liver, legumes and some fish, and a liver enzyme called FMO3 normally converts it into an odorless compound. If that enzyme does not work well, trimethylamine builds up and leaves the body in breath, sweat and urine.

Fishy breath can also be a plain mouth smell. Cleveland Clinic notes that bad breath related to longer-term problems may smell like fish or rotten eggs.

1. Recent fish or seafood. Oily fish, fish sauce and fish oil supplements can leave a smell that lasts hours. If it disappears within a day, this is the answer.
2. Bacteria in the mouth, tonsils or sinuses. Tongue coating, gum disease, tonsil stones and postnasal drip all produce foul odors that people describe in different ways, including fishy.
3. Trimethylaminuria (TMAU). Uncommon, often noticed from childhood or puberty, and episodic. Stress, sweating, periods and certain foods can make it worse. Carriers of one gene change may have milder or temporary episodes.
4. Advanced kidney disease. MedlinePlus lists ammonia-like, urine-like or fishy breath as a sign of chronic kidney failure. Researchers think ammonia, trimethylamine and dimethylamine underlie the fishy note.
5. Liver failure. Its breath is usually described as musty and sweet, but Cleveland Clinic lists trimethylamine as a minor contributor.

There is no cure, but the odor can often be reduced. GeneReviews, the genetic reference used by clinicians, describes diagnosis by a urine test that measures how much trimethylamine is excreted unchanged, together with genetic testing, rather than by smell. Management usually involves a planned diet that limits seafood, eggs, offal, legumes, soya and some brassica vegetables, plus acid soaps and lotions and avoiding situations that cause heavy sweating; doctors sometimes add other treatments. GeneReviews stresses that the diet must still meet choline and folate needs and that choline should not be restricted during pregnancy or breastfeeding, so do not cut out food groups on your own.

MedlinePlus notes the odor can affect relationships, work and mental health. If it is affecting yours, say so to your doctor, because support is part of treatment.

If the smell seems to come only from your mouth, start with two weeks of thorough cleaning: brush twice a day, clean between teeth, clean the back of your tongue once a day, and drink enough water. Treat nasal congestion. If the fishy smell is gone, the source was oral. If it persists, or you also notice it on your skin or in your urine, see a doctor and mention both, since that points away from the mouth. You can also try our bad breath self-check to organize your symptoms before an appointment.