Bad-smelling urine usually comes down to something you ate, how much water you drank, or an infection. In most cases, it’s harmless and temporary. But certain odors can signal a health issue worth paying attention to, especially when paired with other symptoms like pain or fever.
Dehydration Is the Most Common Cause
When you’re not drinking enough water, your urine becomes more concentrated. The waste products that are always present in urine, like urea and ammonia, become packed into a smaller volume of liquid. The result is darker urine with a stronger, more pungent smell. This is the single most frequent reason for noticeably smelly urine, and it resolves quickly once you rehydrate.
Foods That Change Urine Odor
Asparagus is the most famous culprit. Within a few hours of eating it, your body breaks down a compound called asparagusic acid into a mix of sulfur-containing chemicals, including the same compound that gives natural gas its rotten-egg smell. Up to six different sulfur byproducts have been identified in post-asparagus urine. Not everyone notices the smell, though. Some people lack the ability to detect these compounds, so they assume it doesn’t happen to them.
Coffee also changes urine scent noticeably. After you drink it, certain aromatic compounds survive digestion and end up in your urine at elevated concentrations. Two in particular, guaiacol and a compound related to body odor called 3-methylbutanoic acid, increase measurably after coffee consumption. If you’ve ever noticed a distinct smell after your morning cup, that’s why.
Other common dietary triggers include garlic, onions, curry, and cruciferous vegetables like Brussels sprouts and cabbage. These all contain sulfur compounds that your body processes and excretes through urine.
Urinary Tract Infections
A UTI is one of the most common medical causes of foul-smelling urine. Bacteria that colonize the urinary tract produce their own chemical byproducts as they feed on nutrients in your urine, and those byproducts create the smell. The type of odor depends partly on the type of bacteria involved. Proteus species, for example, break down an amino acid called methionine and produce sulfur compounds like methyl mercaptan and dimethyl disulfide, both of which have a strong, unpleasant smell. E. coli, the most common UTI-causing bacterium, ferments sugars and produces ethanol as a byproduct.
Smell alone isn’t enough to diagnose a UTI, but it becomes more suspicious when combined with burning during urination, a frequent urge to go, cloudy or discolored urine, fever, chills, or back pain. A standard urine dipstick test checks for white blood cells and nitrites, both of which indicate bacterial activity.
Vitamins, Supplements, and Medications
B vitamins are well known for changing urine odor and color. Vitamin B6 gives urine a distinct musky scent, while supplements high in thiamin (vitamin B1) or choline produce a similar effect. If you recently started a B-complex supplement or a multivitamin and noticed a change, that’s likely the explanation. Certain chemotherapy drugs can also alter urine smell significantly.
Diabetes and Ketone Buildup
When your body can’t use blood sugar for energy, either because of insufficient insulin or severe insulin resistance, your liver starts breaking down fat as an alternative fuel source. This process produces acids called ketones, which build up in the blood and spill into urine. The result is a distinctive sweet or fruity smell. This happens most dramatically during diabetic ketoacidosis (DKA), a serious complication of diabetes that requires emergency treatment. The same fruity scent often appears on the breath as well.
A milder version of this can happen during prolonged fasting, very low-carb diets, or intense exercise without adequate food intake. In those cases, the ketone levels are lower and typically not dangerous, but your urine may still carry that sweet, acetone-like odor.
Genetic Conditions
Trimethylaminuria, sometimes called fish odor syndrome, is a rare inherited condition where the body can’t break down a chemical called trimethylamine. Normally, an enzyme called FMO3 processes trimethylamine after your body produces it from foods like fish, eggs, and beans. People with this condition inherit mutated copies of the gene responsible for that enzyme from both parents. Without a functioning version, trimethylamine accumulates and seeps into urine, sweat, breath, and saliva, producing a strong fishy or rotten smell.
Maple syrup urine disease is another genetic disorder, this one affecting how the body processes certain amino acids. It produces a distinctive sweet, maple-syrup smell in urine and is typically detected in infancy.
Pregnancy
Many pregnant people report that their urine smells different or stronger, but the explanation is more nuanced than it seems. Research has consistently failed to confirm that pregnant women actually develop a heightened sense of smell in measurable tests, despite the widespread belief. Estrogen levels rise throughout pregnancy and were long assumed to sharpen olfactory sensitivity, but studies comparing pregnant and non-pregnant women on odor identification tasks have found no significant general difference.
That said, pregnancy does involve real changes that affect urine. Prenatal vitamins are loaded with B vitamins and choline, both of which alter urine scent. Hormonal shifts change how your body processes certain foods. And pregnancy increases the risk of UTIs because of changes in urinary tract anatomy and urine composition. So the smell change is real, but the cause is often something specific and identifiable rather than a vague hormonal effect.
Liver and Kidney Problems
Your liver and kidneys are your body’s main filtration systems. When either one isn’t working properly, waste products that would normally be processed or diluted can accumulate and change the way urine smells. Liver disease can produce a musty, ammonia-heavy odor, while kidney dysfunction may cause urine to smell unusually strong because the kidneys can’t concentrate and filter waste effectively. These are less common causes, but persistent changes in urine odor that don’t have an obvious dietary or medication explanation are worth investigating.
What the Smell Can Tell You
The character of the odor offers clues. A strong ammonia smell usually points to dehydration or concentrated urine. A sulfur or rotten-egg smell often traces back to food, especially asparagus, garlic, or cruciferous vegetables. A sweet or fruity smell suggests ketones, which could mean uncontrolled diabetes, fasting, or a very low-carb diet. A fishy smell that persists across multiple days, especially if it also affects your sweat or breath, could indicate trimethylaminuria. A generally foul or unusual smell paired with pain, cloudiness, or fever points toward infection.
A one-time change after a meal or a new supplement is almost never a concern. Persistent changes lasting more than a few days, or smells accompanied by pain, blood, fever, or changes in urination patterns, are the ones that warrant a closer look.