Urine naturally has a mild odor from waste products your kidneys filter out of your blood, but a noticeably strong or unusual smell usually comes down to a handful of causes: what you ate, how much water you drank, an infection, or less commonly, a metabolic condition. Most of the time, smelly urine is temporary and harmless.
Dehydration Is the Most Common Cause
When you’re not drinking enough water, your kidneys still need to flush the same amount of waste, just in less liquid. That concentrates everything, especially urea, a nitrogen-rich compound your body produces constantly as it breaks down protein. High levels of urea give urine a sharp ammonia smell. You’ll also notice the color is darker yellow or amber instead of pale straw.
This is the simplest explanation and the first one to rule out. If your urine smells strong and looks dark, drink more water over the next few hours. The smell should fade as your urine dilutes back to a lighter color.
Foods That Change the Smell
Asparagus is the most famous culprit. Your body breaks down a compound called asparagusic acid into a family of sulfur-containing molecules, the most prominent being methanethiol (also called methyl mercaptan). It’s the same type of chemical that gives rotten eggs and natural gas their distinctive stink. Several other sulfur byproducts show up too, including dimethyl sulfide and dimethyl disulfide, all of which end up in your urine within minutes of eating asparagus.
Here’s the interesting part: about 60% of people can’t smell these compounds at all. A large study of nearly 7,000 people found that three in five lacked the specific olfactory receptors needed to detect the odor. So if you’ve always thought asparagus doesn’t affect your urine, it probably does. You just can’t tell.
Garlic also produces sulfur metabolites in urine, including allyl methyl sulfide, allyl methyl sulfoxide, and allyl methyl sulfone. Of these, only allyl methyl sulfide has a noticeable smell, and researchers have found its influence on overall urine odor is actually minimal because the inherent smell of urine tends to overpower it. So garlic changes urine chemistry, but you’re unlikely to notice it the way you would with asparagus.
Other foods commonly linked to urine odor include onions, certain spices like curry and cumin, and cruciferous vegetables like Brussels sprouts and cabbage. These all contain sulfur compounds that your body processes and excretes.
B Vitamins and Supplements
If you take a multivitamin or B-complex supplement, you may have noticed your urine turns bright yellow and takes on an unusual smell. B vitamins, particularly B2 (riboflavin) and B6, are water-soluble, meaning your body excretes whatever it doesn’t need. That excess ends up in your urine quickly, changing both its color and scent. Other water-soluble supplements can do the same thing. The effect is harmless and fades once the vitamins clear your system.
Urinary Tract Infections
A foul or unusually strong urine odor is one of the hallmark signs of a urinary tract infection. The bacteria responsible, most commonly E. coli, produce specific compounds as they multiply in your urinary tract. Research has identified two particularly smelly metabolites in women with E. coli UTIs: trimethylamine (which has a fishy odor) and putrescine (which smells like decaying flesh). Some strains also produce cadaverine, another foul-smelling compound, especially under stress conditions that help them colonize the bladder.
Smelly urine alone doesn’t confirm a UTI. The combination of odor with burning or pain during urination, cloudy or bloody urine, frequent urges to pee, or fever and back pain points more strongly toward infection. If you have any of those additional symptoms, it’s worth getting tested. A simple urine culture can identify the bacteria involved.
Diabetes and Ketones
When your body can’t use glucose for energy, either because of uncontrolled diabetes or prolonged fasting, it starts burning fat instead. That process produces ketones, which are acids that build up in your blood and spill into your urine. Ketones give urine (and breath) a distinctive sweet or fruity smell that’s quite different from the usual ammonia sharpness of concentrated urine.
In people with diabetes, this can signal diabetic ketoacidosis, a serious and potentially life-threatening condition where ketone levels climb high enough to make the blood dangerously acidic. The fruity smell is often accompanied by excessive thirst, frequent urination, nausea, confusion, and abdominal pain. This is a medical emergency, not something to wait out.
Liver Disease
Severe liver dysfunction produces a characteristic sweet, musty smell in both breath and urine. This is caused by the buildup of dimethyl disulfide and methyl mercaptan, sulfur compounds that a healthy liver would normally process and clear. The smell arises from an excess of the amino acid methionine that the damaged liver can no longer metabolize properly. This odor, known clinically as fetor hepaticus, typically appears only in advanced liver disease and comes alongside other obvious symptoms like jaundice, swelling, and fatigue.
Fish Odor Syndrome
A persistently fishy smell in urine, sweat, and breath can point to trimethylaminuria, a rare genetic condition sometimes called fish odor syndrome. People with this condition lack a functional version of a liver enzyme that normally converts trimethylamine, a fishy-smelling compound, into an odorless form. Without that enzyme, trimethylamine accumulates and gets excreted through all body fluids.
The condition is inherited in an autosomal recessive pattern, meaning you need a copy of the mutated gene from both parents. It’s rare, but because the smell can be socially devastating, it’s worth knowing about if you or someone you know has a chronic fishy body odor that doesn’t respond to hygiene changes. Dietary adjustments that reduce intake of certain nutrients (found in eggs, fish, and legumes) can help lower trimethylamine production.
When the Smell Signals a Problem
A one-time change in urine odor after eating asparagus or taking vitamins is nothing to worry about. What matters is the pattern. Persistent foul-smelling urine, especially when paired with other symptoms like pain, fever, blood, or unusual color, points to something your body needs help resolving. Sweet or fruity urine in someone with diabetes needs immediate attention. A musty or fishy smell that won’t go away despite dietary changes warrants investigation. The smell itself is your body’s signal. Pay attention to what comes with it.