Fishy-smelling urine usually comes from one of a handful of causes, ranging from something you ate to an infection or, less commonly, a metabolic condition. In most cases, the smell is temporary and harmless. But when it persists or comes with other symptoms like burning, fever, or unusual discharge, it points to something worth investigating.
Diet Is the Most Common Culprit
Seafood, especially saltwater fish, is naturally rich in a compound called trimethylamine oxide. When your body digests it, some of that compound converts to trimethylamine, which has a distinctly fishy smell. Your liver normally processes trimethylamine into an odorless form, but if you’ve eaten a large portion of fish, your system may not convert all of it before some ends up in your urine.
It’s not just fish. Foods high in choline, like eggs, liver, and certain legumes, can produce the same effect because gut bacteria convert choline into trimethylamine. Supplements can trigger it too. In one documented case, a woman taking 6 grams per day of L-carnitine (a supplement sometimes used for neurological conditions) developed a persistent, strong fish odor within days of starting treatment. Her liver’s processing system was functional but simply overwhelmed by the dose. Even at normal supplement levels, some people notice a mild fishy quality to their urine.
If diet is the cause, the smell typically clears within a day or two after the meal or supplement is out of your system. Drinking more water helps dilute your urine and reduce the intensity.
Bacterial Vaginosis
For people with vaginas, bacterial vaginosis (BV) is one of the most common reasons urine takes on a fishy smell. BV isn’t a sexually transmitted infection; it’s an overgrowth of certain bacteria that naturally live in the vagina, which shifts the normal bacterial balance. The hallmark symptom is a milky white or gray discharge with a fishy odor. Because urine passes over vaginal tissue on the way out, it picks up that smell, making it seem like the urine itself is the source.
Some people with BV also notice a burning sensation when they urinate. Others have no symptoms at all. BV is treatable with a short course of antibiotics, and the fishy smell resolves once the bacterial balance is restored.
Urinary Tract Infections
UTIs can change the smell of your urine in several ways, including giving it a fishy or foul quality. Bacteria in the urinary tract break down compounds in urine and release byproducts that alter its odor. A UTI is more likely if the smell comes alongside burning during urination, a frequent urge to go, cloudy or dark urine, fever, chills, or back pain. UTIs are straightforward to diagnose with a urine test and respond well to antibiotics.
Trimethylaminuria (Fish Odor Syndrome)
If the fishy smell in your urine is persistent, not tied to what you eat, and also shows up in your sweat or breath, you may have a rare genetic condition called trimethylaminuria. People with this condition have a faulty version of a liver enzyme that normally converts trimethylamine into an odorless compound. Without that conversion, trimethylamine builds up in the blood and comes out in urine, sweat, and breath.
The condition is inherited in an autosomal recessive pattern, meaning you need two copies of the altered gene (one from each parent) to develop symptoms. Its exact prevalence is unknown, but it’s considered uncommon. Some people carry one copy of the gene and have a milder version, where the smell only appears after eating trigger foods rich in choline or carnitine.
There’s no cure, but dietary management makes a significant difference. Reducing intake of choline-rich foods (eggs, organ meats, certain fish, soybeans) and avoiding large doses of carnitine supplements can keep trimethylamine levels low enough that the odor becomes barely noticeable or disappears entirely.
Dehydration
When you’re not drinking enough water, your kidneys have less fluid to dilute the waste products in your urine. The result is darker, more concentrated urine with a stronger smell. This doesn’t always produce a specifically fishy odor, but if there’s even a mild amount of trimethylamine present from your diet, dehydration can amplify it from unnoticeable to obvious. Simply increasing your fluid intake often resolves the issue within hours.
Liver Problems
Your liver is responsible for filtering waste compounds out of your blood, including trimethylamine and other volatile byproducts of metabolism. When the liver isn’t functioning well, these substances build up and come out in your breath, sweat, and urine. Advanced liver disease produces a characteristic smell called fetor hepaticus, which is primarily garlicky or sulfurous, but trimethylamine can contribute a fishy note as well.
Liver-related odor changes are almost always accompanied by other, more noticeable symptoms: fatigue, jaundice (yellowing of the skin or eyes), abdominal swelling, or dark urine. A fishy urine smell on its own, without these other signs, is unlikely to indicate liver disease.
Pregnancy
Pregnant people sometimes notice stronger or different urine odors, but this isn’t always because the urine itself has changed. In early pregnancy, many people develop a heightened sense of smell called hyperosmia, which can make the faint ammonia naturally present in urine suddenly noticeable. Prenatal vitamins, especially those containing B6, can also alter urine odor. Dehydration during pregnancy, which is common due to increased fluid needs, concentrates the urine further.
A genuinely fishy smell during pregnancy is worth mentioning to your provider, since pregnant people are more prone to both UTIs and BV, either of which could be responsible.
How to Tell What’s Causing It
Start with the simplest explanations. Think about what you ate in the last 24 hours and whether you’ve been drinking enough water. If the smell appeared after a seafood meal or a new supplement and fades within a day or two, diet is the likely answer.
If the smell persists beyond a couple of days, or if it comes with burning, unusual discharge, fever, chills, or back pain, an infection is more likely and a urine test can confirm it quickly. A fishy smell that has been present for weeks or months, especially one that also affects your sweat or breath, warrants testing for trimethylaminuria, which involves a simple urine analysis measuring the ratio of trimethylamine to its odorless form.