Smelling feces when there’s no actual source is a real phenomenon called phantosmia, the perception of an odor that isn’t physically present. About 5% of adults experience phantom smells, and fecal or rotting odors are among the most commonly reported types. The causes range from minor and temporary to serious enough to warrant medical attention, so understanding the full picture matters.
What’s Happening in Your Nose and Brain
Your sense of smell depends on a chain of events: odor molecules enter your nose, hit specialized nerve cells in the lining of your nasal cavity, and those cells send electrical signals to your brain for interpretation. A phantom smell can originate from a problem at any point in that chain. If the nerve cells in your nose are damaged or irritated, they can misfire and send signals even when no odor molecules are present. If the issue is in the brain itself, the processing centers that interpret smell signals can generate a fecal odor on their own.
Doctors distinguish between two related conditions. Phantosmia means smelling something that doesn’t exist at all. Cacosmia, a subtype, specifically refers to perceiving a foul odor like feces, rot, or chemicals when nothing is there. Both fall under the umbrella of parosmia, which covers any distortion of normal smell perception. Women are more likely to experience phantom smells than men.
Sinus and Upper Respiratory Causes
The most common triggers are inflammation and infection in or near your nasal passages. Chronic sinusitis, nasal polyps, and lingering upper respiratory infections can all damage or irritate the delicate smell-sensing nerve cells that line your nasal cavity. When those cells are injured, they sometimes regrow incorrectly, creating faulty wiring that sends the wrong signals to your brain. The result can be a persistent or recurring foul smell that has no external source.
If you’ve recently had a cold, sinus infection, or allergies, that’s the most likely explanation. The phantom smell often resolves on its own once the inflammation clears, though it can linger for weeks or months if nerve damage occurred.
COVID-19 and Viral Infections
COVID-19 brought widespread attention to smell distortions. The virus doesn’t directly attack the smell-sensing neurons themselves but instead destroys the support cells surrounding them. When those support cells fail to recover quickly, the neurons die off too. As new neurons grow back, the reconnection process is imperfect, and previously pleasant smells can register as fecal, rotten, or chemical.
Parosmia after COVID often appears with a delayed onset, showing up weeks or months after the initial infection rather than during it. The good news: roughly 95% of people recover their normal sense of smell within six months. Most notice substantial improvement within the first two weeks. Recovery can continue for at least two years after infection, and the regenerative capacity of the smell lining is strongest in younger people.
Medications That Can Trigger Phantom Smells
Several common drug classes are linked to phantom odor perception, particularly in adults over 60. A large population-based study found that diabetes medications, cholesterol-lowering drugs, and acid reflux medications (proton pump inhibitors) each increased the odds of phantom smells by 74% to 88%. Antidepressants, pain medications, anti-seizure drugs, blood pressure medications, anti-anxiety drugs, sedatives, and muscle relaxants have also been associated with higher rates of phantom odors.
If you started or changed a medication around the time the phantom smell appeared, that connection is worth exploring with your prescriber. The smell may resolve with a dosage adjustment or a switch to a different drug in the same class.
Neurological Causes
Less commonly, phantom smells point to something happening in the brain. Temporal lobe epilepsy is the most well-known neurological cause. In these cases, the phantom smell acts as a seizure aura, a brief sensory experience that precedes or accompanies a seizure. The typical smell reported in temporal lobe epilepsy is burning leather, but fecal and other foul odors occur too. A key distinguishing feature: these episodes are brief, repetitive, and may come with other symptoms like a sudden wave of fear, a feeling of déjà vu, or a few seconds of staring and unresponsiveness.
Phantom smells can also be an early feature of Parkinson’s disease, sometimes appearing before the more recognizable motor symptoms like tremor or stiffness. Between 0.5% and 18% of Parkinson’s patients experience olfactory hallucinations, with women making up a disproportionate share. Other neurodegenerative conditions and, rarely, brain tumors can also produce phantom smells. One case report documented a patient whose only initial symptoms of a high-grade brain tumor were phantom smells and taste distortion, with no headaches, vision changes, or balance problems at all.
When the Smell Signals Something Serious
Most phantom fecal smells trace back to benign causes like sinus issues, a past viral infection, or medication side effects. But certain accompanying symptoms raise the stakes:
- Seizure-like episodes: staring spells, sudden confusion, involuntary movements, or a sense of déjà vu paired with the smell
- New headaches that are persistent, worsening, or different from any you’ve had before
- Vision or hearing changes
- Numbness, weakness, or balance problems
- Memory difficulties that are new or worsening
Episodic changes in smell that come and go in short bursts are especially worth investigating, as they can represent seizure auras that need evaluation for infections, tumors, or vascular problems in the brain.
How Doctors Figure Out the Cause
The diagnostic process usually starts with a detailed history: when the smell started, how often it occurs, how long each episode lasts, whether it’s in one nostril or both, and what other symptoms are present. A history of recent respiratory infections, head injuries, migraines, or seizures all help narrow the possibilities.
Nasal endoscopy, where a thin camera is passed through the nose, allows doctors to look directly at the nasal cavity for signs of inflammation, polyps, discharge, or masses near the smell-sensing area. If a sinus problem is suspected, a CT scan of the sinuses provides more detail. For neurological concerns, an MRI of the brain is the standard next step. Formal smell testing using standardized kits (the most common are the Sniffin’ Sticks test and the University of Pennsylvania Smell Identification Test) can measure how well your sense of smell is actually functioning and help pinpoint whether the problem is in the nose or the brain.
Smell Training to Retrain Your Nose
For phantom smells caused by nerve damage, particularly after viral infections, smell training is the most evidence-supported rehabilitation approach. The protocol is straightforward: you deliberately sniff specific scents for about 10 seconds each, with 10-second breaks between them, for a total of about 5 minutes. You do this twice a day, ideally before breakfast and before bed.
A modified version of this training rotates through 12 different scents over 36 weeks. The first 12 weeks use eucalyptus, lemon, clove, and rose. The next 12 weeks switch to menthol, thyme, tangerine, and jasmine. The final 12 weeks introduce green tea, bergamot, rosemary, and gardenia. Rotating the scents prevents your recovering nerve cells from adapting to just a few stimuli and encourages broader olfactory reconnection. Essential oil kits designed for this purpose are widely available.
The training works by encouraging damaged olfactory neurons to regrow correctly and form proper connections. It requires patience and consistency, but it remains the most effective non-medical intervention for persistent smell distortions.