Urine that smells like onions is almost always caused by sulfur-containing compounds, either from something you ate, concentrated waste products from not drinking enough water, or less commonly, a bacterial infection. The smell is usually harmless and temporary, but a persistent onion-like odor paired with other symptoms can signal something worth investigating.
Sulfur Compounds From Food
The most common explanation is your diet. Onions, garlic, shallots, leeks, and other foods in the allium family are rich in sulfur-based compounds. When your body breaks these down, it produces byproducts like allyl methyl sulfide, which has been detected in urine within four hours of garlic ingestion. These compounds are absorbed into your bloodstream and filtered out through your kidneys, essentially making your urine carry the same sulfurous signature as the food you ate.
Cruciferous vegetables like broccoli, Brussels sprouts, and cabbage contain similar sulfur compounds and can produce the same effect. Cumin and certain curry spices are other common culprits. The smell typically fades within 24 to 48 hours after you stop eating the offending food. If you recently had a meal heavy in any of these ingredients, that’s likely your answer.
Dehydration Concentrates the Smell
All urine contains ammonia and trace sulfur compounds as natural waste products. When you’re well hydrated, these are diluted enough that you barely notice them. But when you’re not drinking enough water, everything becomes more concentrated. The ammonia smell in particular can become quite strong, and any sulfurous compounds from food or normal metabolism get amplified alongside it.
You can gauge your hydration by urine color. Pale yellow to nearly clear means you’re well hydrated. Dark amber or honey-colored urine is a reliable sign you need more fluids. Increasing your water intake for a day or two is often enough to resolve the issue if dehydration is the cause.
Supplements and Medications
Several supplements can change how your urine smells. Extra vitamin B6 (pyridoxine) is known to give urine a strong, sometimes pungent odor. High-dose B1 (thiamine) can do the same. If you recently started a new multivitamin or B-complex supplement, that’s worth considering.
On the medication side, sulfa drugs (sulfonamide antibiotics) can give urine a noticeable stench. Certain medications for diabetes and rheumatoid arthritis also affect urine odor. If the smell appeared around the same time you started a new prescription, the timing is probably not a coincidence.
Bacterial Infections
Urinary tract infections can produce sulfurous, foul-smelling urine because the bacteria involved generate their own volatile compounds. Proteus mirabilis, a common UTI-causing bacterium, produces dimethyl disulfide and methyl mercaptan from an amino acid called methionine. These are the same types of sulfur compounds responsible for the smell of onions and rotting vegetables.
A UTI rarely causes smell alone. Other symptoms typically include a burning sensation when you urinate, a persistent urgent need to go, passing only small amounts at a time, cloudy or discolored urine, and pelvic pain or pressure. If the infection reaches the kidneys, you may also develop back pain, fever, chills, or nausea. The presence of any of these symptoms alongside the odor change points toward an infection that needs treatment.
Sweat From the Groin Area
Sometimes the smell isn’t coming from your urine at all. Your groin has a high concentration of apocrine sweat glands, the type that secrete an oily fluid containing proteins, lipids, and steroids. Bacteria on your skin break down this sweat into pungent compounds. One in particular, a thioalcohol produced by Staphylococcus hominis, smells distinctly like rotten onions. Because the groin is close to where you urinate, it’s easy to assume the smell is in your urine when it’s actually on your skin.
If the onion smell seems to persist regardless of what you eat or how much water you drink, try showering with a gentle cleanser before checking your urine odor again. Wearing breathable, moisture-wicking underwear can also reduce bacterial buildup in the area.
Pregnancy and Hormonal Changes
Pregnant people often report that their urine smells different or stronger, but the change is frequently perceptual rather than chemical. In early pregnancy, many people develop hyperosmia, a heightened sense of smell that can make faint odors suddenly noticeable. Ammonia and sulfur compounds that were always present in urine may seem new or stronger simply because your nose has become more sensitive to them.
That said, pregnancy does involve real changes too. Prenatal vitamins, particularly those high in B6, can alter urine odor directly. Dietary cravings and aversions may shift what you eat in ways that affect the smell. And pregnancy increases the risk of UTIs, so a new or persistent odor during pregnancy is worth mentioning at your next appointment.
When the Smell Persists
An onion smell that lasts a day or two after a specific meal, during a stretch of low water intake, or right after starting a new supplement is almost certainly benign. The smell should resolve once the trigger is gone. If it persists for more than a few days despite good hydration and no obvious dietary cause, or if it’s accompanied by pain, burning, frequent urination, fever, or blood in your urine, those are signs of a possible infection or other urinary tract issue that warrants a urine test. A simple urinalysis can identify or rule out infection quickly.