Bad-smelling urine is usually harmless and temporary, most often caused by something you ate, drank, or how hydrated you are. In some cases, though, a strong or unusual odor signals an infection or an underlying health condition worth paying attention to. The smell of your urine reflects what your body is filtering out, so changes in diet, hydration, medications, and metabolism all show up in the toilet bowl.
Dehydration Is the Most Common Cause
When you’re not drinking enough water, your kidneys concentrate waste products into a smaller volume of urine. That concentrated urine has a stronger ammonia smell and darker yellow color. This is by far the most frequent reason for noticeably smelly urine, and the fix is straightforward: drink more fluids until your urine returns to a pale straw color. If the smell goes away once you’re well-hydrated, there’s nothing else going on.
Foods That Change Urine Smell
Asparagus is the most well-known dietary culprit. Your body breaks down a compound called asparagusic acid into a family of sulfur-containing chemicals, including methanethiol, dimethyl sulfide, and dimethyl disulfide. These are the same types of volatile molecules that give rotten eggs and cooked cabbage their distinctive odors, and they pass into your urine within 15 to 30 minutes of eating asparagus. Not everyone can smell it. Roughly 40% of people lack the specific smell receptors needed to detect these compounds, so the change may be happening without you noticing.
Other foods that can alter urine odor include garlic, onions, Brussels sprouts, curry, and large amounts of coffee. These effects are harmless and disappear within a day or two once the food clears your system.
Urinary Tract Infections
A foul or unusually strong urine smell, especially one that’s new and doesn’t go away with hydration, can point to a urinary tract infection. Bacteria in the urinary tract break down amino acids and other compounds in ways that produce volatile, smelly byproducts. Proteus species, for example, convert the amino acid methionine into methyl mercaptan and dimethyl disulfide, both of which have a strong sulfurous stench. E. coli causes roughly 70% of community-acquired UTIs and 50% of hospital-acquired ones, with Proteus, Klebsiella, and Staphylococcus species making up much of the rest.
A UTI rarely shows up as smell alone. Look for burning or pain during urination, a frequent urgent need to pee (sometimes producing very little), cloudy or pinkish urine, and lower abdominal pressure. If the infection involves the kidneys, you may also have fever, chills, or back pain. These symptoms together with a change in urine odor are a clear signal to get a urine test.
Medications, Vitamins, and Supplements
Several medications can change how your urine smells. Antibiotics, particularly sulfonamide-type drugs, are well-known for giving urine a sulfur-like odor. B vitamins are another common cause. They can give urine both a greenish tint and a noticeable smell, especially when taken in high-dose supplements. The change is a sign that your body is excreting what it doesn’t need and is not harmful.
If you recently started a new medication or supplement and notice a change in urine smell, check the label for listed side effects. The odor typically persists as long as you’re taking the supplement and resolves when you stop.
Diabetes and Ketones
A sweet or fruity urine smell can be a sign that your body is producing excess ketones. This happens when cells can’t get enough glucose for energy and start burning fat instead. The fat-burning process releases ketone bodies, including acetone, the same chemical found in nail polish remover. Acetone has a distinctive sweet, fruity odor that shows up in both breath and urine.
For people with diabetes, this smell can indicate diabetic ketoacidosis, a serious complication where blood becomes dangerously acidic. It’s most common in type 1 diabetes but can occur in type 2 as well. Other signs include excessive thirst, frequent urination, nausea, abdominal pain, and confusion. This is a medical emergency.
People following very low-carb or ketogenic diets also notice a fruity urine smell. In this context, it simply means the body is in ketosis and burning fat. The smell is more of a nuisance than a danger for otherwise healthy people, though staying well-hydrated helps reduce it.
Liver Disease
Advanced liver disease can produce a distinct musty, pungent odor in urine, sweat, and breath. This smell, sometimes called fetor hepaticus, comes primarily from two sulfur compounds: dimethyl sulfide, which has a garlicky quality, and methyl mercaptan, which smells like rotten eggs or cabbage. Healthcare providers have described the overall effect as oddly sweet and occasionally fecal.
This odor develops because a failing liver can no longer properly filter sulfur-containing amino acids from the blood. It’s a late-stage sign and would almost always accompany other symptoms of liver disease, such as jaundice (yellowing of the skin or eyes), abdominal swelling, persistent fatigue, and easy bruising.
Rare Genetic Conditions
Two inherited metabolic disorders are closely associated with distinctive urine smells, though both are uncommon.
Trimethylaminuria (Fish Odor Syndrome)
Trimethylaminuria causes urine, sweat, and breath to smell strongly of fish. It’s caused by mutations in a gene called FMO3, which provides instructions for an enzyme that normally converts a fishy-smelling compound called trimethylamine into an odorless form. When both copies of this gene are dysfunctional, trimethylamine builds up and is excreted unchanged. Diagnosis involves collecting a urine sample to measure trimethylamine levels, sometimes after eating a large portion of fish to challenge the enzyme system. Genetic testing can confirm specific mutations. There’s no cure, but reducing intake of foods high in choline, such as eggs, liver, and certain legumes, can significantly reduce the odor.
Maple Syrup Urine Disease
This condition gets its name from the sweet, maple syrup-like smell of affected infants’ urine. It results from the body’s inability to break down three amino acids (leucine, isoleucine, and valine) found in protein-rich foods like milk, meat, and eggs. These amino acids and their byproducts accumulate in the blood and urine. It’s typically detected through newborn screening and managed with a carefully controlled diet from infancy.
Patterns Worth Paying Attention To
A single episode of strong-smelling urine after asparagus, coffee, or a day of not drinking enough water is nothing to worry about. The patterns that matter are persistent changes: urine that smells foul for several days despite good hydration, or a new sweet or fishy odor that doesn’t match anything in your diet. Smell combined with other symptoms tells you more than smell alone. Pain or burning with urination, fever, blood in the urine, or back pain alongside a foul odor points toward infection. A fruity smell paired with unusual thirst and frequent urination suggests a blood sugar problem.
Your nose is surprisingly useful as an early warning system. Most of the time it’s picking up something mundane, but when the smell is new, persistent, and unexplained, it’s giving you information worth following up on.