Urine that smells like fish usually points to a bacterial infection, a dietary trigger, or dehydration. In most cases the cause is straightforward and treatable, but a persistent fishy smell, especially paired with other symptoms, can signal something that needs medical attention.
Urinary Tract Infections
A UTI is the most common reason urine takes on a fishy or foul odor. Bacteria, most often E. coli from the digestive tract, enter the urethra and multiply in the bladder. As they break down waste products in urine, they release chemicals that produce that distinctive smell. Women are significantly more likely to develop UTIs because the urethra is shorter and closer to the rectum, making bacterial migration easier.
Along with the odor change, UTIs typically cause a burning sensation when you pee, a frequent urgent need to urinate even when your bladder is nearly empty, and urine that looks cloudy or slightly pink. Some people also feel pressure or cramping in the lower abdomen. If the infection reaches the kidneys, you may develop a fever, back pain, nausea, or chills. A simple course of antibiotics clears most bladder infections within a few days, and the smell resolves as the bacteria die off.
Bacterial Vaginosis
For women and people with vaginas, bacterial vaginosis (BV) is another frequent culprit. BV develops when the normal balance of vaginal bacteria shifts, allowing certain species to overgrow. The hallmark symptom is a thin, grayish-white discharge with a strong fishy smell, and that odor easily transfers to urine or is noticeable during urination. The smell often becomes more intense after sex.
BV is not a sexually transmitted infection, though sexual activity can disrupt the bacterial balance enough to trigger it. Douching, new sexual partners, and using scented soaps in the genital area all increase risk. It sometimes resolves on its own, but prescription treatment shortens the duration and reduces the chance of complications like pelvic inflammatory disease.
Dehydration and Diet
When you’re not drinking enough water, your urine becomes more concentrated. That concentration intensifies whatever compounds are already present, and the result can be a strong, unpleasant smell that some people describe as fishy. Dark yellow or amber-colored urine alongside the odor is a reliable sign that dehydration is the issue. Increasing your fluid intake for a day or two should dilute the urine enough to eliminate the smell.
Certain foods also change how urine smells. Asparagus is the most well-known offender, but fish itself (especially tuna and other oily varieties), Brussels sprouts, garlic, onions, and coffee can all alter urine odor. These food-related changes are harmless and temporary, usually clearing within 24 to 48 hours after you stop eating the trigger food. B6 supplements and some medications can do the same thing.
Trimethylaminuria
A rare but notable cause is trimethylaminuria, sometimes called “fish odor syndrome.” People with this genetic condition can’t properly break down trimethylamine, a compound produced during digestion of foods rich in choline, carnitine, and nitrogen. Choline-heavy foods include eggs, liver, legumes, and saltwater fish. Instead of being processed and excreted without odor, trimethylamine builds up and is released through urine, sweat, and breath, producing a persistent fishy smell.
The condition is present from birth, though symptoms can range from barely noticeable to severe depending on how much residual enzyme activity a person has. Dietary modifications, specifically limiting high-choline foods, are the primary way to manage the smell. Some people find that low doses of certain antibiotics reduce the gut bacteria responsible for producing trimethylamine in the first place. If you’ve noticed a lifelong pattern of fishy-smelling body fluids that doesn’t match any infection or dietary explanation, this condition is worth discussing with a doctor. A urine test measuring trimethylamine levels can confirm the diagnosis.
Sexually Transmitted Infections
Trichomoniasis, a common STI caused by a parasite, can produce a fishy-smelling discharge that mingles with urine. In women, it often causes frothy yellow-green discharge, genital itching, and discomfort during urination or sex. Men with trichomoniasis frequently have no symptoms at all, which makes transmission easy. Chlamydia and gonorrhea can also cause unusual-smelling discharge, though the odor is less characteristically “fishy” and more generally foul.
If the smell appeared after a new sexual partner or is accompanied by discharge, itching, or pain, STI testing is a reasonable next step. All three of these infections are curable with appropriate treatment.
Pregnancy and Hormonal Changes
Pregnancy heightens your sense of smell, which can make normal urine odor seem stronger or more unpleasant than usual. But pregnancy also genuinely changes urine composition. Hormonal shifts alter vaginal pH, making pregnant women more susceptible to both UTIs and BV, both of which cause fishy-smelling urine. Prenatal vitamins, particularly those high in B vitamins, can contribute to odor changes as well.
Menopause brings its own set of changes. Declining estrogen thins the vaginal and urethral tissues, disrupts the normal bacterial environment, and increases vulnerability to infections. A new or worsening fishy urine smell during perimenopause or menopause is often tied to these shifts.
Kidney and Liver Problems
Less commonly, a persistent change in urine odor can reflect how well your kidneys or liver are functioning. The kidneys filter waste from the blood, and when they’re not working efficiently, waste products accumulate and alter urine smell. Liver disease can impair the body’s ability to process toxins, sometimes producing a musty or fishy odor in urine. These causes are uncommon and almost always accompanied by other symptoms like fatigue, swelling, appetite loss, or changes in skin color. Odor alone, without other warning signs, rarely points to organ dysfunction.
What the Smell Is Telling You
A one-time fishy smell after eating salmon or on a day you didn’t drink enough water is nothing to worry about. The smell should clear within a day or two once you hydrate or the food passes through your system. If the odor persists for more than two or three days without an obvious dietary explanation, or if it comes with burning, discharge, fever, or pain, an underlying infection is the most likely cause and worth getting checked.
Color offers a useful clue alongside smell. Clear to pale yellow urine with an unusual odor is less concerning than dark, cloudy, or discolored urine with a strong smell. Blood-tinged urine with a foul odor always warrants prompt evaluation. A simple urinalysis can identify most infections within minutes and guide treatment quickly.