Loss of smell has dozens of possible causes, but most cases trace back to a handful of common culprits: upper respiratory infections (including COVID-19), nasal obstruction, head injuries, aging, and neurological conditions. The medical term for complete smell loss is anosmia, while partial loss is called hyposmia. Understanding what’s behind your specific case matters because some causes resolve on their own, others need treatment, and a few signal something more serious.
How Your Sense of Smell Works
Smell starts when airborne molecules enter your nose and land on a thin layer of tissue called the olfactory mucosa, located high in your nasal cavity. Specialized receptor cells in this tissue detect those molecules and send electrical signals through nerve fibers that pass through a thin piece of bone called the cribriform plate, then into the olfactory bulb at the base of your brain. From there, the signal travels deeper into the brain, where it’s interpreted as a specific scent.
Anything that disrupts this chain, from a physical blockage in the nose to damage in the brain itself, can reduce or eliminate your ability to smell. Even chewing food or sipping a drink releases molecules that travel up the back of your throat to reach these same receptors, which is why smell loss also dulls your sense of taste.
Upper Respiratory Infections
The common cold, flu, and sinus infections are the most frequent cause of temporary smell loss. Inflammation swells the nasal passages and blocks odor molecules from reaching the olfactory receptors. In most cases, smell returns within a week or two as the infection clears. Chronic sinusitis, where inflammation persists for 12 weeks or longer, can cause more stubborn loss that lasts until the underlying condition is treated.
How COVID-19 Damages Smell
COVID-related smell loss works differently from a typical cold. The virus doesn’t primarily attack the smell-detecting neurons themselves. Instead, it targets the support cells surrounding them, called sustentacular cells. These cells express the receptor (ACE2) that SARS-CoV-2 uses to enter human tissue, making them a direct target for infection.
Sustentacular cells do critical behind-the-scenes work: they regulate the chemical environment neurons need to function, produce protective compounds, and help maintain the tiny hair-like structures (cilia) on the neurons that actually detect odor molecules. When the virus damages these support cells, the neurons lose their cilia and their ability to properly relay scent signals to the brain, even though the neurons themselves are largely uninfected.
During recovery, the genes that encode odor receptors can reactivate in altered patterns. This flawed recombination of signals is thought to explain parosmia (distorted smells, like coffee smelling like garbage) and phantosmia (smelling things that aren’t there), both of which are common in the months after a COVID infection. Most people recover their smell within weeks to a few months, but a subset experiences dysfunction lasting a year or longer.
Nasal Polyps and Structural Blockages
Nasal polyps are soft, painless, noncancerous growths that form in the lining of the nose or sinuses. When they grow large enough, they physically block the nasal passages and prevent odor molecules from reaching the olfactory receptors. Polyps tend to develop in people with chronic allergies, asthma, or recurring sinus infections. They’re typically diagnosed with a nasal scope or CT scan, and treatment ranges from steroid sprays that shrink the polyps to surgical removal in stubborn cases.
A deviated septum or other structural abnormalities can create a similar blockage effect, though these are less commonly the sole cause of significant smell loss.
Head Injuries
A blow to the head, even from a mild traumatic brain injury, can shear the delicate nerve fibers where they pass through the cribriform plate. This bony structure has small perforations that the olfactory nerve fibers thread through, and the force of impact can stretch or sever them. Frontal or occipital (back of the head) impacts carry the highest risk. Unlike smell loss from a cold, post-traumatic anosmia is often permanent because nerve fibers in this region regenerate slowly and unreliably. Some people experience partial recovery over months, but complete restoration is uncommon after severe shearing injuries.
Aging
Smell naturally declines with age, a process sometimes called presbyosmia. The numbers are striking: about 13% of Americans aged 60 to 69 have some form of smell impairment. That jumps to 25% for those in their 70s and 39% for people 80 and older. This gradual decline results from the slow loss of olfactory receptor cells and reduced regeneration capacity in the nasal lining. It’s typically so gradual that many older adults don’t realize it’s happening until it’s fairly advanced.
This matters beyond just missing pleasant aromas. Smell loss in older adults increases the risk of poor nutrition (food becomes less appealing), accidental poisoning, and failure to detect gas leaks or smoke.
Neurological Conditions
Smell loss is one of the earliest warning signs of both Parkinson’s disease and Alzheimer’s disease, often appearing years before the more recognizable symptoms like tremor or memory loss. In Parkinson’s specifically, roughly 55% of patients already have significant smell impairment by the time they’re diagnosed. Research published in the journal Brain found that Parkinson’s patients with severe smell loss had a dramatically higher risk of progressing to dementia: 41.7% developed dementia within three years, compared to 0% of Parkinson’s patients whose sense of smell was still intact.
This doesn’t mean that losing your sense of smell means you have Parkinson’s. The vast majority of smell loss cases are caused by infections, allergies, or aging. But persistent, unexplained smell loss, especially combined with other subtle changes like sleep disturbances or constipation, is worth mentioning to a doctor.
Chemical and Environmental Exposure
Long-term exposure to certain industrial chemicals can damage or destroy olfactory tissue. Cadmium, formaldehyde, certain solvents, and heavy metals like arsenic are known offenders. Some of these substances react directly with the olfactory receptor cells or form chemical compounds that block signal transmission. This type of damage tends to be cumulative and, in many cases, irreversible. Workers in manufacturing, welding, and chemical processing face the highest risk, particularly when working without adequate ventilation or respiratory protection.
Cigarette smoking also dulls smell over time, though this is at least partially reversible after quitting.
Medications
Certain medications can interfere with smell as a side effect. Some antibiotics, blood pressure medications, and chemotherapy drugs are known to affect olfactory function. The loss is usually temporary and resolves after stopping the medication, though chemotherapy-related changes can linger. If you notice smell changes after starting a new prescription, it’s worth flagging for your prescriber, as an alternative may be available.
Olfactory Training for Recovery
For people whose smell hasn’t bounced back on its own, olfactory training is the most widely recommended rehabilitation approach. The protocol is simple: you sniff four distinct scents (typically rose, lemon or orange, eucalyptus, and clove) for about five minutes, twice a day. Each scent represents a different odor category, and the repetition is thought to stimulate the regeneration and rewiring of olfactory pathways.
Consistency matters more than anything. Studies show that sticking with the protocol for three months or longer produces significantly better results than shorter attempts. Combining olfactory training with supplements like vitamin A has shown additional benefits in some research. Improvement tends to be gradual rather than sudden, so patience is part of the process. Most people who improve notice changes in scent detection thresholds first, with accurate scent identification following later.
Olfactory training works best for post-infectious smell loss, including COVID-related cases. It’s less effective when the cause is structural (polyps, deviated septum) or neurological, since those require treating the underlying condition directly.