Urine that smells like onions is almost always caused by sulfur-containing compounds, whether from something you ate, not drinking enough water, or bacteria on the skin near your urethra. The onion-like smell is rarely a sign of anything serious, but understanding the source helps you figure out whether it will resolve on its own or deserves a closer look.
Sulfur-Rich Foods Are the Most Common Cause
Onions, garlic, shallots, leeks, and other plants in the allium family are packed with organosulfur compounds. When you eat garlic, for example, an enzyme called alliinase converts odorless compounds in the clove into allicin the moment the cells are crushed. Your body then metabolizes allicin through a chain of reactions, ultimately producing a compound called allyl methyl sulfide. This sulfide, along with two related metabolites, gets filtered through your kidneys and excreted in urine.
Research published in Frontiers in Nutrition tracked these garlic-derived metabolites in human urine and found that concentrations peaked about one to two hours after eating garlic, with a second spike appearing six to eight hours later in some people. That second wave explains why your urine can still smell pungent the morning after a garlic-heavy dinner. Onions follow a similar metabolic path because they share the same sulfur chemistry. Cruciferous vegetables like Brussels sprouts, broccoli, and cabbage also contain sulfur compounds that can alter urine odor, though the smell is typically less sharp than what alliums produce.
If your diet recently included any of these foods, that’s the most likely explanation. The smell should fade within 24 to 48 hours as the sulfur metabolites clear your system.
Dehydration Concentrates the Smell
When you’re not drinking enough water, your kidneys produce less urine with a higher concentration of waste products. That means the same sulfur compounds, urea, and other metabolic byproducts are packed into a smaller volume of liquid, making any existing odor much stronger. You might eat the same garlic pasta you always do but only notice the onion-like smell on days when you’ve been under-hydrated.
Darker yellow urine is the easiest visual clue. If your pee is pale straw-colored, you’re well-hydrated and any dietary sulfur compounds will be diluted enough that the smell is faint or undetectable. If it’s amber or honey-colored and has a strong odor, increasing your water intake for a day or two is often all it takes to resolve it.
Sweat and Bacteria Near the Groin
Sometimes the onion smell isn’t coming from urine at all. It’s coming from the skin around your genitals, and you notice it when you use the bathroom. Apocrine sweat glands, which are concentrated in the groin, armpits, and scalp, secrete an oily fluid made of proteins, lipids, and steroids. This fluid is odorless on its own, but skin bacteria break it down into volatile compounds that can smell distinctly like onions.
One bacterium in particular, Staphylococcus hominis, produces a thioalcohol called 3M3SH that smells like rotten onions or meat. According to research from the American Society for Microbiology, S. hominis imports a precursor molecule into its cells and converts it into this pungent compound. Because the groin is warm, moist, and home to dense bacterial colonies, the onion-like odor can be strong enough that it seems like it’s in the urine itself. Washing the area with mild soap and wearing breathable underwear can help you tell the difference. If the smell disappears after a thorough shower but your urine still smells neutral, the source was your skin, not your kidneys.
Medications and Supplements
Several medications and vitamins alter urine odor in ways that can read as sulfurous or onion-like. Sulfa drugs, including sulfonamide antibiotics, are among the most common culprits. Excess vitamin B6 (pyridoxine) can also give urine a strong, unusual smell. Medications used for diabetes and rheumatoid arthritis have been linked to changes in urine odor as well.
If you recently started a new medication or supplement and the smell appeared around the same time, the timing is probably not a coincidence. The odor typically persists as long as you’re taking the medication and resolves once you stop or once your body adjusts.
Less Common Metabolic Causes
In rare cases, persistently unusual urine odor can point to a metabolic condition. Cystinuria is a genetic disorder where the kidneys excrete excessive amounts of certain amino acids, including cystine, which contains sulfur. Healthy kidneys excrete about 40 to 80 milligrams of cystine per day; people with cystinuria excrete significantly more. The condition is primarily known for causing recurrent kidney stones rather than odor alone, and it’s diagnosed through urine testing that detects characteristic hexagonal crystals.
Uncontrolled diabetes can also change the way urine smells, though it more commonly produces a sweet or fruity odor from ketones rather than an onion-like one. Urinary tract infections sometimes cause a foul or ammonia-heavy smell, but the hallmark symptoms of a UTI (burning, urgency, cloudy urine) are usually more noticeable than the odor itself.
How to Narrow Down the Cause
Start with the simplest explanation. Think back over the past 12 to 24 hours: did you eat onions, garlic, or cruciferous vegetables? Have you been drinking less water than usual? Did you start any new supplements? In most cases, one of these will be the answer.
If the smell persists for more than a few days after ruling out diet and hydration, or if it’s accompanied by pain, cloudiness, blood in the urine, or fever, that’s worth bringing to a doctor. A basic urinalysis can quickly screen for infection, excess amino acids, or other abnormalities. But for the vast majority of people who notice an onion-like smell, the fix is as straightforward as drinking more water and waiting for last night’s dinner to finish working through their system.