Hyposmia, a reduced ability to detect odors, affects people far more often than most realize and stems from a surprisingly wide range of causes. Upper respiratory infections, chronic sinus disease, head injuries, neurological conditions, toxic exposures, and simple aging can all blunt the sense of smell. Because the olfactory system is one of the few parts of the nervous system that continuously regenerates its own neurons, some forms of hyposmia improve on their own or respond to treatment, while others prove stubbornly persistent. The path from noticing that food tastes bland to figuring out why and what to do about it involves smell-specific testing, sometimes imaging, and treatments that range from nasal steroids to structured scent-retraining exercises.
How the Sense of Smell Works and Where It Breaks Down
Odor molecules enter the nose, dissolve in the mucus lining of the olfactory cleft, and bind to receptors on olfactory sensory neurons embedded in a small patch of tissue called the olfactory epithelium. Those neurons fire signals through the skull into the olfactory bulb, a structure that relays information to several brain regions. Unlike every other sense, smell bypasses the thalamus and projects directly to the primary olfactory cortex, which includes the piriform cortex, amygdala, and entorhinal cortex.1Clinical and Experimental Otorhinolaryngology. The Olfactory System: Basic Anatomy and Physiology for General Otorhinolaryngologists That direct wiring partly explains why smells trigger vivid memories and strong emotions in ways that sights and sounds often do not.
A disruption anywhere along this chain causes trouble. If swollen tissue or polyps physically block odor molecules from reaching the sensory neurons, the problem is conductive. If the neurons themselves are damaged, or if the brain regions that interpret their signals are impaired, the problem is sensorineural. Many cases of hyposmia involve both mechanisms at once.
The olfactory epithelium is unusual in that it houses its own population of stem cells. Globose basal cells handle day-to-day neuron replacement, while horizontal basal cells sit dormant until a more serious injury wakes them up to rebuild the tissue.2PubMed Central. Stem Cell Niches for Olfactory Regeneration and Their Therapeutic Applications This regenerative capacity is the reason many people recover smell after a cold or even a moderate head injury. But it also has limits: when the stem cells themselves are damaged or when chronic inflammation keeps disrupting the repair process, recovery stalls.
Viral and Post-Infectious Causes
Upper respiratory viruses have always been one of the leading triggers of lasting smell loss, and COVID-19 put the issue squarely in public view. The SARS-CoV-2 virus does not primarily attack olfactory neurons directly. Instead, it infects the sustentacular cells, the support cells that nourish and maintain the sensory neurons, because those cells express the ACE2 receptor the virus uses to enter.3PubMed Central. Post-viral olfactory loss and parosmia When those support cells go down, the sensory neurons lose their lifeline. Research in animal models and human tissue samples shows widespread damage to the hair-like cilia on olfactory neurons and a drop in the expression of the receptor proteins that actually detect odors.4PubMed Central. SARS-CoV-2 infection of sustentacular cells disrupts olfactory signaling pathways
For most people with COVID-related smell loss, the virus clears within a few weeks. But in a subset, T-cell-driven inflammation lingers in the olfactory tissue long after the virus itself is gone, and the number of functioning sensory neurons stays reduced.5PubMed Central. Persistent post-COVID-19 smell loss is associated with immune cell infiltration and altered gene expression in olfactory epithelium This ongoing immune response, rather than residual viral damage, appears to be the key mechanism behind prolonged post-COVID hyposmia. Other viruses, including influenza and parainfluenza, can cause similar post-viral smell loss through comparable inflammatory pathways, though the specific cell targets differ.
Chronic Rhinosinusitis and Allergies
Chronic rhinosinusitis, with or without nasal polyps, is among the most common causes of persistent hyposmia. The mechanism is a one-two punch. First, swollen mucosa and polyps physically block odor molecules from reaching the olfactory cleft. Second, the inflammation itself infiltrates the olfactory epithelium and damages the sensory neurons directly.6PubMed Central. Olfactory Dysfunction in Patients with Chronic Rhinosinusitis The severity of smell loss tends to track with how much inflammatory damage the neuroepithelium has sustained, not just how congested the nose feels.
In chronic rhinosinusitis driven by type 2 inflammation, the mucosal swelling and polyp growth in the olfactory cleft can cause shedding and degeneration of the olfactory epithelium itself.7Journal of Allergy and Clinical Immunology. Hyposmia: Causes, Diagnosis, and Treatment Options This means that even after polyps are removed or swelling is reduced, some patients still have diminished smell because the underlying tissue has been remodeled. Allergic rhinitis operates along similar lines on a milder scale, with persistent inflammation gradually eroding olfactory function over months or years.
Head Trauma
A blow to the head can damage the sense of smell in two ways. A fracture or swelling near the nose or sinuses can create a conductive blockage, which sometimes responds to surgery. More often, the delicate olfactory nerve fibers that pass through tiny holes in the skull base get sheared or stretched. Because those fibers are extremely thin and the bony plate they cross is perforated like a sieve, even a moderate impact can sever them. The prognosis for post-traumatic smell loss is sobering: roughly only a third of patients show improvement over time.8PubMed Central. Head trauma and olfactory function Injuries to the back of the head are particularly risky for smell loss because of the way the brain shifts inside the skull during a contrecoup impact.
Neurological Disease and Aging
Hyposmia is one of the earliest detectable signs of Parkinson’s disease, often appearing years before the tremors and movement difficulties that lead to a diagnosis. A meta-analysis pooling several studies found that people with hyposmia had roughly four times the risk of developing Parkinson’s compared to those with a normal sense of smell.9PubMed Central. Hyposmia as a Predictive Marker of Parkinson’s Disease: A Systematic Review and Meta-Analysis Alzheimer’s disease follows a similar pattern, with smell loss preceding cognitive decline. In both conditions, the same protein aggregates that damage brain cells elsewhere also accumulate in the olfactory bulb and cortex early in the disease course.
Even without a neurological disease, the sense of smell declines with age. Research into the olfactory epithelium of older adults has found changes in the gene expression of the stem cells responsible for neuron replacement, suggesting the regenerative machinery gradually winds down.10JCI Insight. Aging-related olfactory loss is associated with olfactory stem cell transcriptional alterations in humans This age-related decline is gradual enough that many older adults do not notice it until it becomes significant, which makes routine smell screening in older populations a topic of growing clinical interest.
Toxic Exposures and Medications
Workplace and environmental chemical exposures can damage olfactory tissue directly. Epidemiological investigations have identified occupational exposure to cadmium, chromium, nickel, and formaldehyde as established causes of olfactory impairment.11PubMed Central. Olfactory dysfunction revisited: a reappraisal of work-related olfactory dysfunction caused by chemicals Case reports also implicate ammonia, gasoline fumes, hairdressing chemicals, chemotherapy agents, and intranasal zinc products.12PubMed Central. Toxin-induced chemosensory dysfunction: A case series and review The zinc-containing nasal sprays deserve special mention because they were sold as cold remedies before being linked to permanent smell loss and eventually pulled from the U.S. market.
Nutritional factors play a subtler role. Zinc is involved in maintaining the special senses, and there is limited evidence that zinc deficiency impairs olfactory function in humans.13PubMed. Zinc and the special senses However, zinc supplementation has not proven to be a reliable treatment for smell disorders broadly. A controlled trial found that while patients with taste and smell dysfunction did show abnormalities in zinc metabolism, the results did not support zinc supplementation as a blanket therapeutic approach.14American Journal of the Medical Sciences. A double blind study of the effects of zinc sulfate on taste and smell dysfunction
Congenital and Genetic Forms
Some people are born with a diminished or absent sense of smell. Congenital hyposmia has been linked to deficiencies in the adenylyl cyclase signaling pathway that olfactory neurons depend on.15The FASEB Journal. Initiation of olfactory memory in patients after successful treatment of patients with congenital hyposmia – the role of neuroplasticity Congenital anosmia, the complete absence of smell from birth, can occur as an isolated condition or as part of a genetic syndrome. A systematic review identified over 80 candidate genes and chromosomal regions associated with congenital olfactory dysfunction, though most of them are linked to syndromic forms where smell loss accompanies other developmental features.16PubMed. Genetics of congenital olfactory dysfunction: a systematic review of the literature Isolated congenital anosmia remains genetically heterogeneous and underdiagnosed, largely because people who have never had a sense of smell often do not realize what they are missing until well into adulthood.
How Hyposmia Is Diagnosed
A surprising number of people with meaningful smell loss underestimate their problem or attribute it to aging. Formal testing catches what self-report misses. The most widely used clinical tools are psychophysical tests: you sniff a series of standardized scents and try to identify, discriminate, or detect them at decreasing concentrations. The Sniffin’ Sticks battery, for example, measures odor threshold, discrimination, and identification as three separate scores that are combined into a composite.17PubMed. Sniffin’Sticks: a new olfactory test battery The University of Pennsylvania Smell Identification Test (UPSIT) is another common option, particularly in North America, using scratch-and-sniff cards. Cutoff scores are adjusted for age because normal olfactory ability declines across decades.18PubMed. Malaysian Version of the Sniffin’ Sticks Identification Smell Test: Cutoff Points of Hyposmia
Objective tests that measure brain or nerve responses to odor stimuli exist but remain largely confined to research settings. They provide precise and standardized results, yet their cost and the expertise required to administer them limit their everyday clinical use.19PubMed. Comparative Review of Olfactory Assessment Methods
MRI can add useful information, particularly when a neurological or structural cause is suspected. The olfactory bulb, visible on MRI, tends to shrink when it receives less input over time, and its volume correlates with olfactory test scores.20PubMed Central. MR Imaging-Based Evaluations of Olfactory Bulb Atrophy in Patients with Olfactory Dysfunction Beyond the bulb itself, brain imaging studies have found gray matter reductions in regions like the orbitofrontal cortex, insular cortex, and piriform cortex in patients with olfactory loss from various causes.21PubMed. Magnetic Resonance Imaging as a Diagnostic and Research Tool in Patients with Olfactory Dysfunction: A Systematic Review These brain changes illustrate the “use it or lose it” principle: areas of the brain that stop receiving smell input can physically shrink.
Corticosteroids and Medical Treatment
Corticosteroids are the most commonly prescribed medication for hyposmia, but their effectiveness depends entirely on the cause. In chronic rhinosinusitis and allergic rhinitis, where inflammation is the primary culprit, nasal corticosteroids make a real difference. Studies of mometasone furoate and fluticasone furoate in persistent allergic rhinitis showed statistically significant improvements in olfactory function after about four weeks of use.22PubMed Central. Effects of corticosteroids on hyposmia in persistent allergic rhinitis In these patients, reducing the inflammatory swelling reopens the conductive pathway and allows damaged tissue to begin healing.
For post-viral smell loss, steroids are far less reliable. Clinical guidelines note that double-blind, randomized, placebo-controlled trials of several drugs for post-viral olfactory dysfunction have failed to demonstrate statistically significant benefits.23Auris Nasus Larynx. Clinical practice guidelines for the management of olfactory dysfunction — Secondary publication A meta-analysis of six trials looking specifically at corticosteroids for post-COVID smell loss found no significant improvement in self-rated smell scores, though objective testing at the end of treatment showed slightly higher scores in the steroid group, and the odds of recovering from complete smell loss were modestly higher.24PubMed Central. The Effect of Corticosteroids on Post-Covid-19 Smell Loss: A Meta-Analysis The evidence suggests steroids may help in the acute, reversible stage of post-viral olfactory injury but offer little once the damage has settled in.
Olfactory Training
Olfactory training is the closest thing to a broadly recommended rehabilitation strategy for smell loss. The standard protocol involves sniffing four distinct scents, traditionally rose, eucalyptus, lemon, and cloves, twice a day for at least three months. It sounds almost too simple, but the approach has real evidence behind it. In COVID-related parosmia and hyposmia, patients who did a modified version of this training showed significantly greater improvement at three, six, and nine months compared to a control group. Extending the training from six to nine months produced additional gains.25PubMed Central. Modified Olfactory Training Is an Effective Treatment Method for COVID‐19 Induced Parosmia
The mechanism appears to work on two levels. At the periphery, the repeated stimulation may encourage the regeneration of olfactory sensory neurons. Centrally, neuroimaging studies suggest that the consistent sensory input drives structural and functional changes in the brain’s olfactory processing pathways.26PubMed Central. Olfactory Training for COVID-19-Related Olfactory Dysfunction: A Systematic Review and Meta-Analysis of Randomized Controlled Trials Patience is essential; improvements tend to be gradual, and many clinicians recommend committing to the practice for at least four to six months before judging whether it is working.
Surgery for Sinus-Related Smell Loss
When chronic rhinosinusitis with nasal polyps fails to respond to medical therapy, endoscopic sinus surgery can dramatically improve smell. A meta-analysis of olfactory outcomes after sinus surgery found significant improvement across multiple smell tests in patients with mixed chronic rhinosinusitis, with polyp patients experiencing the largest gains.27PubMed. Olfactory Outcomes after Endoscopic Sinus Surgery for Chronic Rhinosinusitis: A Meta-analysis One study of patients with bilateral nasal polyposis found that before surgery, 85% were anosmic and 15% were hyposmic. After surgery, normal smell returned in over half the patients.28PubMed Central. Impact of endoscopic sinus surgery on smell sensation and nasal airway resistance in patients with chronic rhinosinusitis with bilateral nasal polyposis
There is an interesting nuance in the data: patients who were completely anosmic before surgery showed the most dramatic score improvements, while those who started with hyposmia did not reach statistically significant gains in some studies.29PubMed Central. Does Olfactory Function Improve After Endoscopic Sinus Surgery? This likely reflects a ceiling effect; hyposmic patients had less room to improve on the scoring scales, and their baseline function was already partially intact. Surgery is not a treatment for post-viral, post-traumatic, or neurological smell loss because there is no obstructive disease to remove in those cases.
Emerging Therapies
Platelet-rich plasma injections into the olfactory cleft represent one of the more intriguing experimental approaches. PRP contains growth factors that could theoretically support nerve regeneration. A pilot study found that among hyposmic patients who received PRP, 60% achieved normal smell scores at three months.30PubMed Central. The use of platelet‐rich plasma in treatment of olfactory dysfunction: A pilot study A subsequent randomized controlled trial in patients with COVID-related smell loss found that PRP treatment produced a roughly 3.7-point greater improvement in smell scores than placebo at three months, and the response rate was substantially higher in the PRP group.31PubMed Central. Use of platelet-rich plasma for COVID-19-related olfactory loss: a randomized controlled trial These results are promising but come from small studies, and PRP for smell loss is not yet a standard treatment. Larger trials are needed before it can be broadly recommended.
Recovery Patterns and Prognosis
For COVID-related smell loss specifically, a significant majority recover early. One analysis placed the prevalence of early olfactory recovery at about 63%, with a median time to complete recovery of two weeks.32PubMed. Prevalence and Prognostic Factors Associated with Early Recovery of Olfactory Dysfunction in COVID-19 Patients However, certain factors predict slower or less complete recovery. A meta-analysis found that women were roughly half as likely as men to recover their sense of smell after COVID, and greater initial severity of the dysfunction was also associated with poorer outcomes. Nasal congestion at onset further reduced recovery odds.33BMJ. Prognosis and persistence of smell and taste dysfunction in patients with covid-19: meta-analysis with parametric cure modelling of recovery curves
The sex difference in recovery is consistent across multiple studies and remains somewhat unexplained, though hormonal influences on immune response and olfactory tissue are among the hypotheses. Patients who also lost their sense of taste had somewhat worse smell recovery compared to those with isolated smell loss.32PubMed. Prevalence and Prognostic Factors Associated with Early Recovery of Olfactory Dysfunction in COVID-19 Patients For post-traumatic hyposmia, as noted earlier, the outlook is less optimistic, with only about a third improving. For sinus-related smell loss, the prognosis is generally the best of any category, provided the underlying inflammation is controlled.
Daily Life, Safety, and Psychological Impact
Living with hyposmia carries consequences that go well beyond missing the aroma of coffee. Depressive symptoms accompany olfactory disorders in anywhere from about a third to three-quarters of patients, depending on the cause, and the relationship follows a dose-dependent pattern: the worse the smell loss, the more pronounced the depressive symptoms.34PubMed Central. The impact of olfactory loss on quality of life: a 2025 review The pathways linking smell loss to depression run in two directions. One is experiential: food loses its appeal, social eating becomes joyless, and worries about body odor lead some people to withdraw from social situations entirely. The other is neurological: olfactory loss physically changes how the brain processes emotional stimuli.
Safety is a more tangible concern. A cross-sectional study found that over 85% of patients with olfactory disorders reported high levels of safety anxiety, and over a five-year period, about a third experienced at least one food-related incident from being unable to detect spoilage, while roughly 15% had at least one gas-related incident.35PubMed Central. Impact of olfactory disorders on personal safety and well-being: a cross-sectional observational study People with smell loss develop compensatory habits, such as checking expiration dates compulsively, never leaving the stove unattended, or relying on family members to sniff-test food. Some install extra smoke and gas detectors. These adaptations are practical and worth adopting early.
Eating behaviors shift unpredictably. Some people with hyposmia eat less because food is no longer appetizing, risking weight loss and nutritional deficiency, particularly in older adults. Others eat more, reaching for salty, sugary, or fatty foods to compensate for the missing flavor dimension.34PubMed Central. The impact of olfactory loss on quality of life: a 2025 review Neither pattern is healthy long-term, and dietitian involvement can help patients whose eating has changed substantially after losing their sense of smell.
Why Humans Are Already Working with a Reduced Olfactory System
The human genome carries a striking number of broken olfactory receptor genes. A genomic comparison across species found that humans have accumulated mutations disrupting their olfactory receptor coding regions roughly four times faster than any other species examined. As a result, the fraction of olfactory receptor pseudogenes (genes that have been effectively switched off by mutations) in humans is almost twice as high as in other primates.36PubMed Central. Human specific loss of olfactory receptor genes This accelerated gene loss is thought to reflect a reduced dependence on smell as human ancestors came to rely more heavily on vision and social cognition. We are, in a sense, already working with a pared-down olfactory toolkit compared to our closest relatives, which makes the remaining functional capacity all the more worth protecting when disease, injury, or environmental exposure threatens it.