Smelling urine when there’s no actual source is a type of phantom smell, known medically as phantosmia. About 5% of adults experience phantom smells, and urine or ammonia is one of the more commonly reported varieties. The causes range from mild and temporary (a lingering sinus infection) to more significant (kidney problems or neurological conditions), so understanding the pattern and context of your symptom matters.
What Phantom Smells Actually Are
Phantosmia means your brain is registering an odor that isn’t physically present in your environment. This is different from parosmia, where real smells get distorted into something unpleasant. Both can produce a urine-like sensation, but the distinction matters: if you notice the smell only when eating or sniffing something specific, that’s likely parosmia. If it appears randomly, with no trigger at all, that’s phantosmia.
Phantom smells are surprisingly common. In a large population-based study of adults 40 and older, about 6% reported experiencing them. Women are affected more often than men, and the prevalence increases with age. Olfactory hallucinations are actually the most common type of sensory hallucination in healthy people, reported by roughly 8.6% of participants in one multi-country study, with 3.5% experiencing them at least monthly.
Sinus and Upper Respiratory Causes
The most frequent explanation is also the most mundane. Inflamed or infected sinuses can trap bacteria that produce sulfur and nitrogen compounds, some of which smell like ammonia or stale urine. Chronic sinusitis keeps mucus stagnant in the sinus cavities, and that stagnation creates the perfect environment for odor-producing bacteria. You might notice the smell is worse when you bend forward, wake up in the morning, or blow your nose.
Upper respiratory infections, including common colds and flu, can also damage the delicate smell receptors lining the inside of your nose. When those receptors heal improperly, they may send distorted signals to your brain. This is essentially a wiring problem: neurons that normally detect neutral or pleasant smells get cross-connected with pathways linked to foul odors. The result is a persistent, unpleasant phantom smell that can linger for weeks or months after the original infection clears.
Post-COVID Smell Distortion
COVID-19 made this problem dramatically more visible. Between 28% and 43% of people who lost their sense of smell during a COVID infection went on to develop parosmia, where everyday smells became warped into something chemical, burnt, or urine-like. The mechanism appears to involve incomplete healing of smell receptors. When only some receptor types regenerate, your brain gets a partial picture of an odor. The highly volatile, unpleasant components of a smell come through clearly while the more pleasant notes are missing, creating a lopsided perception that often registers as ammonia, sewage, or urine.
This distortion typically emerges 45 to 60 days after the initial infection, which aligns with the time needed for olfactory neurons to regenerate. The frustrating part is how long it can last. Many patients still report smell distortion at six months, and a significant number at 12 months. The encouraging news is that about 85% of people no longer report these distortions after two years, suggesting the nervous system does eventually complete its repairs for most people.
Kidney Problems and Ammonia Buildup
Your kidneys filter waste products from your blood, including urea. When kidney function declines, urea accumulates in the bloodstream and eventually breaks down into ammonia. This ammonia gets released through saliva, sweat, and breath, creating a persistent smell that you may notice most strongly in your own nose and mouth. In chronic kidney disease, this ammonia odor combines with other waste compounds to produce what’s sometimes described as a fishy or urine-like smell.
If the urine smell in your nose is accompanied by fatigue, swollen ankles, changes in urination frequency, nausea, or a metallic taste in your mouth, kidney function is worth investigating. A simple blood test can measure how well your kidneys are filtering waste.
Neurological Triggers
Phantom smells can originate in the brain rather than the nose. Temporal lobe seizures are one of the more well-documented neurological causes. The temporal lobe processes smell, and abnormal electrical activity there can generate vivid olfactory hallucinations. The classic phantom smell in temporal lobe epilepsy is burning rubber or leather, but urine, chemicals, and other unpleasant odors are also reported. These smell episodes are typically brief (lasting seconds to a couple of minutes), may come with a sense of déjà vu or a rising feeling in the stomach, and can occur in isolation without the convulsions most people associate with seizures.
Other neurological conditions linked to phantosmia include Parkinson’s disease, brain tumors (particularly those near the olfactory areas), and head injuries. Trauma to the head can physically damage the olfactory bulb or the frontal cortex regions that process smell. Imaging studies have shown measurable shrinkage of the olfactory bulb and reduced metabolic activity in smell-processing brain areas following traumatic brain injury.
Other Common Causes
Several less dramatic explanations are worth considering. Dehydration concentrates waste products in your body, and the ammonia in your own breath or nasal mucus may become more noticeable when you’re not drinking enough water. Certain medications, including some antibiotics, blood pressure drugs, and chemotherapy agents, can alter your sense of smell as a side effect. Aging itself changes olfactory function; the smell receptors thin out and regenerate more slowly over time, making phantom and distorted smells more likely after age 60.
Dental infections deserve mention too. The roots of your upper teeth sit very close to your maxillary sinuses. An abscess or deep infection in an upper molar can spread bacteria into the sinus cavity, producing foul-smelling drainage that you perceive as coming from inside your nose. This is easy to overlook because the tooth itself may not hurt if the nerve has died.
Figuring Out the Cause
The pattern of the smell offers useful diagnostic clues. A smell that is constant and worsens with head position points toward a sinus issue. One that comes in brief, sudden episodes lasting under two minutes suggests a neurological origin. A smell that appeared weeks after a respiratory illness fits the post-viral parosmia timeline. And a persistent ammonia-like odor accompanied by other systemic symptoms (fatigue, swelling, appetite changes) raises the question of kidney function.
Doctors can test your sense of smell using standardized scratch-and-sniff identification tests that measure both your ability to detect odors and your accuracy in identifying them. If a neurological cause is suspected, imaging of the brain can check for structural problems. For sinus-related causes, a nasal endoscopy or CT scan of the sinuses reveals inflammation, polyps, or trapped infection. In many cases, a thorough history of when the smell started, how long episodes last, and what other symptoms are present narrows the possibilities considerably before any testing is needed.
What Helps
Treatment depends entirely on the cause. Sinus infections respond to nasal irrigation with saline, steroid nasal sprays, and in some cases antibiotics. If chronic sinusitis is the culprit, regular saline rinses can keep the nasal passages clear and reduce bacterial buildup. For post-viral parosmia, smell training is the most studied approach: you deliberately sniff four distinct scents (commonly rose, lemon, clove, and eucalyptus) for 20 seconds each, twice daily, over several months. This appears to help the olfactory neurons reorganize their connections more accurately.
Phantosmia that stems from a neurological source requires addressing the underlying condition. For temporal lobe epilepsy, controlling seizure activity typically resolves the phantom smells. Medication-induced phantosmia often improves after switching to an alternative drug. When kidney disease is responsible, managing the kidney condition and its progression reduces the ammonia buildup that causes the odor. For cases where no clear cause is found, many people find that the phantom smell fades on its own over months, particularly if it began after a viral illness.