Strong-smelling urine is usually a sign that your urine is more concentrated than normal, most often from not drinking enough water. When fluid intake drops, waste products like urea build up in a smaller volume of urine, producing a noticeable ammonia-like smell. While dehydration is the most common explanation, several other causes range from completely harmless dietary triggers to conditions that deserve medical attention.
Dehydration and Concentrated Urine
Your kidneys filter waste from your blood and dissolve it in water to create urine. When you’re well-hydrated, that waste is diluted and your urine is pale with little odor. When you haven’t had enough fluids, the same amount of waste ends up in much less water. The result is darker yellow urine with a stronger ammonia smell, because urea (one of the main waste products) is present at higher concentrations.
This is by far the most frequent reason for strong-smelling urine, and it resolves quickly once you drink more water. Morning urine tends to be the most concentrated and strongest-smelling, since you haven’t had fluids for several hours overnight. If strong odor is the only thing you notice and it clears up with better hydration, there’s generally nothing else going on.
Foods That Change Urine Odor
Asparagus is the classic example. Your body breaks down a compound called asparagusic acid into sulfur-containing byproducts, the most prominent being methanethiol. These give urine a distinctive sulfurous smell, often compared to cooked cabbage, that can appear within 15 to 30 minutes of eating asparagus.
Interestingly, not everyone notices it. About 6% of people have a specific genetic variation near an olfactory receptor gene (OR2M7) that makes them unable to detect the smell at all, even though it’s present in their urine. There may also be individual differences in how much of the sulfur compound people actually produce after eating asparagus. So if you’ve never noticed asparagus-scented urine, your nose may simply be wired to skip over it.
Other foods known to affect urine smell include garlic, onions, coffee, and certain spices. These changes are temporary and harmless.
Vitamins and Medications
B vitamins are well-known urine changers. Vitamin B6 is particularly associated with giving urine a musky odor, and supplements containing vitamin B1 (thiamin) or choline can have a similar effect. Prenatal vitamins, which are loaded with B vitamins, are a common culprit during pregnancy. Some chemotherapy drugs also alter urine smell significantly.
If you recently started a new supplement or medication and noticed a change, that’s very likely the explanation. The smell typically persists as long as you’re taking the supplement.
Urinary Tract Infections
A urinary tract infection can give urine a foul or fishy smell that’s distinctly different from the ammonia scent of concentrated urine. The bacteria responsible, most commonly E. coli (which causes roughly 80% of UTIs), produce specific compounds as they break down nutrients in the urinary tract. Two of the main odor-causing byproducts are trimethylamine, which has a strong fishy and rancid smell, and putrescine, which smells like rotting fish or decaying animal matter. Both are found at significantly higher concentrations in the urine of people with an active UTI.
Strong-smelling urine alone doesn’t confirm an infection. UTIs typically come with other symptoms: a burning sensation when you urinate, a frequent or urgent need to go, cloudy or discolored urine, and sometimes pelvic pain or low-grade fever. If you notice foul-smelling urine alongside any of these, a simple urinalysis can confirm whether bacteria and white blood cells are present.
Diabetes and Ketones
A sweet or fruity smell to your urine can signal that your body is producing excess ketones. This happens when cells can’t get enough glucose for energy and start burning fat instead. The resulting acids, called ketones, build up in the blood and spill into the urine.
In people with diabetes, this can progress to a dangerous condition called diabetic ketoacidosis, especially when blood sugar climbs above 300 mg/dL. The fruity scent may also be noticeable on your breath. Other warning signs include excessive thirst, frequent urination, nausea, and confusion. This is a medical emergency that requires prompt treatment.
People following very low-carb or ketogenic diets may also notice a mildly sweet or acetone-like urine smell as their body shifts to burning fat. In this context, without diabetes, the smell is generally not dangerous.
Liver Disease
Advanced liver disease can produce a characteristic sweet and musty odor in both urine and breath, known as fetor hepaticus. This smell comes from the buildup of sulfur compounds, specifically dimethyl disulphide and methyl mercaptan, which accumulate when the liver can’t properly process the amino acid methionine. This is a feature of severe liver dysfunction, not early-stage liver problems, and it would appear alongside other significant symptoms like jaundice, abdominal swelling, or fatigue.
Pregnancy
Many pregnant people notice their urine smells stronger, especially in the first trimester. This often has less to do with actual changes in urine chemistry and more to do with a heightened sense of smell called hyperosmia. Ammonia is always present in urine at low levels, but a newly sharpened nose can pick up faint scents that previously went unnoticed.
That said, real changes in urine odor during pregnancy do happen. Shifting food cravings, prenatal vitamins (especially the B6 content), and dehydration from morning sickness all contribute. Pregnant people are also at higher risk for UTIs, so a persistently foul smell is worth mentioning to a healthcare provider.
Rare Genetic Conditions
Trimethylaminuria, sometimes called fish odor syndrome, is a rare inherited condition where the body can’t break down trimethylamine, a compound produced when gut bacteria digest certain nutrients like choline. Normally, a liver enzyme converts trimethylamine into an odorless form. People with this condition lack enough of that enzyme, so trimethylamine accumulates and is released in urine, sweat, and breath, producing a persistent fishy smell.
The condition is inherited in an autosomal recessive pattern, meaning you need to receive the gene variant from both parents. It’s uncommon, but if you’ve had a chronic fishy body or urine odor that doesn’t respond to dietary changes or hygiene, it’s one possibility worth investigating through specialized testing.
What Testing Looks Like
If strong-smelling urine persists and isn’t explained by dehydration, diet, or supplements, a standard urinalysis is the usual first step. This involves three parts: a physical examination that assesses color, clarity, and odor; a chemical analysis using a reagent strip that checks for glucose, protein, ketones, white blood cells, nitrites (a marker of bacterial infection), and bilirubin; and a microscopic examination looking for bacteria, cells, and crystals. Together, these components can identify or rule out infections, diabetes, kidney issues, and liver problems quickly and inexpensively.
Persistent changes in urine odor that come with other symptoms, such as pain, fever, blood in the urine, unusual thirst, or unexplained weight loss, point to something that needs evaluation beyond simple hydration.