Common Causes (Ranked by Likelihood)
UTIs are the #1 cause of fishy-smelling urine. Bacteria (usually E. coli) infecting the urinary tract produce trimethylamine and other sulfur compounds as metabolic byproducts. UTIs affect about 50-60% of women at some point in their lives and are less common but still possible in men.
BV is an imbalance of vaginal bacteria where anaerobic bacteria (especially Gardnerella vaginalis) overgrow and produce trimethylamine. BV discharge can mix with urine during urination, making urine smell fishy. It affects about 29% of women ages 14-49 in the US.
Eating fish (especially ocean fish high in trimethylamine oxide), asparagus, Brussels sprouts, or taking fish oil supplements or choline supplements can cause temporarily fishy urine. Your body metabolizes these compounds and excretes the byproducts through urine. This is completely harmless.
Concentrated urine contains higher levels of urea, ammonia, and other waste products that can produce a strong, sometimes fishy odor. When you're dehydrated, there's less water to dilute these compounds, making the smell much more noticeable.
TMAU is a rare genetic condition where the body can't break down trimethylamine, causing it to accumulate and be excreted through urine, sweat, and breath. Affected individuals have a persistent fish odor that doesn't respond to normal interventions. Estimated to affect fewer than 1 in 100,000 people.
How to Fix It
Treat the Infection
$0-150Most effective — addresses the #1 cause.
- 1See a doctor for a urine culture to confirm UTI and identify the specific bacteria.
- 2Antibiotics (typically 3-7 day course) resolve most UTIs. Common prescriptions: nitrofurantoin, trimethoprim-sulfamethoxazole.
- 3For BV: metronidazole (oral or vaginal gel) for 5-7 days is the standard treatment.
- 4Drink plenty of water during treatment to help flush bacteria.
Hydrate and Adjust Diet
$0For dietary or dehydration causes.
- 1Drink 8+ glasses of water daily — your urine should be pale yellow.
- 2If you suspect fish or supplements, eliminate them for 3-5 days and see if smell resolves.
- 3Cranberry juice (unsweetened) may help prevent UTIs by preventing bacteria from adhering to bladder walls.
- 4Probiotics with Lactobacillus strains may help maintain healthy urinary and vaginal flora.
See a Specialist
$200-500For recurring infections or suspected TMAU.
- 1If UTIs recur 3+ times per year, see a urologist for underlying cause evaluation.
- 2For suspected TMAU, a urine test measuring trimethylamine levels can confirm diagnosis.
- 3TMAU management includes avoiding choline-rich foods (eggs, organ meats, saltwater fish) and taking riboflavin supplements.
- 4A gynecologist can help with recurrent BV — long-term management may include boric acid suppositories.
When to Call a Professional
Fishy urine with fever, back pain, or blood in urine — may indicate kidney infection. Seek urgent care.
Fishy smell persisting more than 48 hours with urinary symptoms — likely UTI needing antibiotics.
Recurrent fishy smell with vaginal discharge — BV needs treatment to prevent complications.
Fishy smell from urine, sweat, AND breath — possible TMAU. See a genetic specialist.
How to Prevent It
Stay well-hydrated — diluted urine has less odor and flushes bacteria.
Urinate after sex — reduces UTI risk by flushing bacteria from the urethra.
Wipe front to back — prevents E. coli from reaching the urethra.
Avoid douching — disrupts normal vaginal flora and increases BV risk.
Wear breathable cotton underwear — reduces moisture that encourages bacterial growth.
Trimethylamine: The Chemistry of Fishy Smell
The compound responsible for fishy smell is trimethylamine (TMA), a nitrogen-containing organic compound with the chemical formula N(CH₃)₃. In its pure form, it has an overwhelmingly fishy, ammoniacal odor detectable at incredibly low concentrations — as little as 1 part per billion.
In the body, TMA is produced when gut bacteria break down choline, carnitine, and trimethylamine N-oxide (TMAO) from food. Normally, an enzyme called FMO3 in the liver converts TMA to odorless TMAO, which is excreted in urine. When infections produce excess TMA, or when FMO3 is deficient (as in TMAU), the smelly form accumulates.
UTI-causing bacteria, particularly certain strains of E. coli and Proteus mirabilis, produce TMA and hydrogen sulfide as metabolic byproducts. This is why infected urine has a characteristically fishy or sulfurous smell — the bacteria are literally producing the same compounds found in rotting fish.
Frequently Asked Questions
No. Diet (fish, asparagus, fish oil supplements), dehydration, and certain medications can cause temporary fishy urine. However, if the smell persists more than 48 hours or is accompanied by pain, urgency, or discharge, see a doctor to rule out UTI or BV.
Yes. While UTIs are more common in women, men can get them too, especially with prostate issues. Dehydration, diet, and the rare condition TMAU affect both sexes equally. Men with persistent fishy urine should see a doctor.
Cranberry may help prevent UTIs (by preventing E. coli from adhering to the bladder wall), but it won't treat an existing infection. Unsweetened cranberry juice or cranberry extract supplements have the most evidence. It won't help with BV or TMAU.
Morning urine is the most concentrated (you haven't had water for 6-8 hours), so any odor is amplified. If it only smells fishy in the morning and resolves after hydrating, dehydration is the likely cause. If it persists throughout the day, consider infection.