Losing your sense of smell, known medically as anosmia, most often results from viral infections, chronic sinus inflammation, head injuries, or the natural decline that comes with aging. In many cases the loss is temporary, but for a significant number of people it persists for months or years, affecting everything from the enjoyment of food to the ability to detect danger. The causes range from straightforward nasal blockages to early signs of serious neurological disease, and understanding which category you fall into shapes what can be done about it.
How Smell Works in the First Place
Airborne chemical molecules enter the nose and bind to specialized olfactory receptors sitting high up in the nasal cavity, on a patch of tissue called the olfactory epithelium. These receptors belong to a large family of genes, and once activated, they convert the chemical signal into an electrical one that travels along olfactory sensory neurons to the olfactory bulb, the brain’s first processing station for smell information.1PubMed Central. Sense of Smell: Structural, Functional, Mechanistic Advancements and Challenges in Human Olfactory Research From there, the signal fans out to several brain regions involved in memory, emotion, and conscious identification of the odor.
What makes this system vulnerable is that it has multiple points of failure. Anything that blocks airflow to the olfactory epithelium, damages the receptor cells themselves, disrupts the nerve connections to the brain, or injures the brain regions that process smell can reduce or eliminate the sense. That is why the list of possible causes is so long.
Post-Viral Smell Loss and COVID-19
Upper respiratory infections have always been one of the leading causes of lasting smell loss. The common cold, influenza, and other viruses can inflame or damage the olfactory epithelium, and for most people smell returns within weeks. But a subset of patients find that it does not come back, or comes back distorted.
COVID-19 brought this problem into sharp focus. Research showed that the SARS-CoV-2 virus primarily infects sustentacular cells, the support cells that sit alongside olfactory sensory neurons in the nasal lining, rather than the neurons themselves.2Cell. Non-neuronal expression of SARS-CoV-2 entry genes in the olfactory system suggests mechanisms underlying COVID-19-associated anosmia The infection triggers local inflammation and immune cell infiltration, which leads to widespread damage to the hair-like cilia on olfactory neurons and a drop in the expression of olfactory receptors and the signaling molecules those neurons need to function.3PubMed Central. SARS-CoV-2 infection of sustentacular cells disrupts olfactory signaling pathways In other words, the neurons do not have to be directly infected to stop working. When their support system is knocked out, they lose function too.
For most people with COVID-related smell loss, the sense returns within a few weeks to months as the epithelium regenerates. But a meaningful number experience prolonged or permanent loss, and many go through a phase of distorted smell on the way back. That distortion has its own name and its own problems, covered further below.
Sinus Disease and Nasal Polyps
Chronic rhinosinusitis, a persistent inflammation of the nasal passages and sinuses, is one of the most common causes of reduced smell. Olfactory dysfunction is so characteristic that it is considered one of the main diagnostic features of the condition.4PubMed Central. Chronic Rhinosinusitis-Related Smell Loss: Medical And Surgical Treatment Efficacy The inflammation can physically block odor molecules from reaching the olfactory epithelium, and in more severe cases it damages the sensory tissue itself.
Nasal polyps, the soft growths that develop from chronically inflamed sinus tissue, make things worse. Patients with chronic rhinosinusitis and nasal polyps have a particularly high rate of anosmia.5PubMed. Early clinical improvement of anosmia and sinus nitric oxide in chronic rhinosinusitis with nasal polyps subjects treated with dupilumab The good news is that sinus-related smell loss tends to respond better to treatment than most other causes. Topical and systemic steroids can reduce the swelling enough to restore airflow and allow odor molecules through, and in cases that do not respond to medication, sinus surgery is an option.6PubMed. Management of Smell Dysfunction Some patients find that their smell returns temporarily with oral steroids but fades again when they stop, a pattern sometimes called steroid-dependent anosmia.7PubMed. Steroid-dependent anosmia
Head Trauma
A blow to the head, particularly one involving the front or back of the skull, can damage the sense of smell in two ways. In the “conductive” type, swelling or structural changes in the nose block airflow to the olfactory area, and this can sometimes be fixed with medication or surgery. In the “neurosensory” type, the fine nerve fibers connecting the olfactory epithelium to the brain are sheared as they pass through the bony plate at the top of the nasal cavity. This kind of damage is much harder to reverse.
The prognosis after traumatic smell loss is generally poor. Only about a third of patients see improvement, and most people with neurosensory damage will not recover their sense of smell.8PubMed Central. Head trauma and olfactory function This makes head-injury-related anosmia one of the more frustrating causes for patients and clinicians alike.
Medications and Toxic Exposures
Hundreds of medications have been reported to alter or reduce the sense of smell and taste, either on their own or in combination with other drugs. The risk tends to increase with the number of prescriptions a person takes, which is one reason smell problems become more common in older adults who are on multiple medications. Most drug-related chemosensory side effects stem from interference with the biochemical signaling pathways that olfactory cells use to detect and transmit odor information.9PubMed Central. Influence of medications on taste and smell
Long-term exposure to certain industrial chemicals, heavy metals, and airborne pollutants can also damage the olfactory epithelium. Unlike medication-related loss, which may reverse if the offending drug is stopped, toxic exposure can cause irreversible harm depending on the substance and duration of contact.
Smell Loss as an Early Warning of Brain Disease
One of the more unsettling aspects of olfactory dysfunction is its link to neurodegenerative disease. Smell impairment is a common and early feature of both Parkinson’s disease and Alzheimer’s disease, and it often appears years before the motor symptoms of Parkinson’s or the memory problems of Alzheimer’s become noticeable.10PubMed. Olfaction as an early marker of Parkinson’s disease and Alzheimer’s disease The reason is that the pathological changes in these diseases, including abnormal protein buildup, begin in the olfactory system before they spread to other brain areas.
This does not mean that every person who loses their sense of smell is developing a neurodegenerative condition. The vast majority of smell loss has other explanations. But olfactory testing has become an area of growing interest in neurology because it offers a potential early diagnostic window. A decline in smell that cannot be explained by sinus disease, infection, or medication, especially in someone over 60, is worth discussing with a doctor.11PubMed Central. Olfactory dysfunction: common in later life and early warning of neurodegenerative disease
Born Without Smell
A small number of people have never been able to smell at all. Isolated congenital anosmia is present from birth and is believed to be genetic. Researchers using whole-exome sequencing on families with the condition have identified candidate genes involved in olfactory signaling, including CNGA2, a gene essential for the pathway that converts a chemical signal into a nerve impulse in the nose.12PubMed Central. Identifying candidate genes underlying isolated congenital anosmia The genetic landscape is complex, with over a hundred candidate genes identified so far, which suggests that multiple different genetic disruptions can produce the same end result.
People with congenital anosmia often report that they did not realize they were missing anything until childhood or adolescence, when the social role of smell became more apparent. Because they have never had the sense, they tend to adapt well in daily life, though they face the same safety risks as anyone else who cannot smell.
Distorted Smell Is Not the Same as No Smell
Not everyone with a smell disorder experiences a simple absence. Parosmia is the distortion of existing smells, so that things that used to smell pleasant now smell foul or strange. Phantosmia, by contrast, is the perception of a smell that is not actually there. Both can be caused by viral infections, head injuries, sinus disease, medications, and neurological or psychiatric conditions.13PubMed Central. Parosmia and Phantosmia: Managing Quality Disorders
Parosmia surged during the COVID-19 pandemic and often appears during the recovery phase of smell loss, typically three months to a year after the initial loss. It tends to be more common in younger women with post-viral smell loss, while phantosmia is more common in middle-aged individuals and those whose smell was affected by head trauma.14PubMed Central. Prevalence and correlates of parosmia and phantosmia among smell disorders Parosmia is generally considered a sign that the olfactory neurons are regenerating but have not yet wired themselves back together correctly. That is small comfort when coffee smells like burning rubber, but it does suggest the system is trying to heal.
Why You Think You Lost Your Taste
One of the most common misconceptions about smell loss is that it means you have also lost your sense of taste. Most people who report losing taste have actually lost smell. What we experience as “flavor” is overwhelmingly driven by smell; the tongue itself detects only sweet, salty, sour, bitter, and umami. When the smell component disappears, food becomes bland and nearly indistinguishable, and it is natural to interpret that as a loss of taste. Truly isolated loss of taste, without any smell involvement, is relatively rare.15PubMed Central. Isolated taste disorders in patients referred to a flavor clinic with taste and smell loss
This distinction matters because it changes where doctors look for the problem. If you report taste loss, a clinician who understands this relationship will first test your smell, because that is far more likely to be the culprit.
Safety Risks That Come With Smell Loss
Losing your sense of smell is not just an inconvenience. It removes an early warning system for real dangers. In a study of patients with anosmia, about 45% had experienced at least one hazardous event related to their inability to smell. The most common were cooking-related incidents, followed by ingestion of spoiled food, inability to detect gas leaks, and inability to smell fire or smoke.16JAMA Otolaryngology–Head & Neck Surgery. Hazardous Events Associated With Impaired Olfactory Function The risk was proportional to the severity of the smell loss, dropping from about 45% in people with complete anosmia to about 24% in those with mild impairment.
A more recent survey found that about 86% of people with olfactory disorders reported being worried about safety, with gas, food freshness, and smoke ranked as the top concerns.17PubMed Central. Impact of olfactory disorders on personal safety and well-being: a cross-sectional observational study Respondents described persistent anxiety, habitual checking of gas valves, and what some called a paranoia about hazards they could not detect. Practical countermeasures, like installing natural gas detectors, being vigilant about food expiration dates, and using smoke alarms with loud alerts, become essential adaptations.
The Emotional Weight of Living Without Smell
Smell loss is often trivialized by people who have never experienced it, but research consistently shows it takes a real toll on mental health. Olfactory impairment is associated with reduced quality of life and can contribute to depression.18PubMed Central. Depression, Olfaction, and Quality of Life: A Mutual Relationship The loss of pleasure in eating is the most obvious impact, but the effects run deeper than that.
Qualitative research with people living with anosmia reveals a pattern of social withdrawal and emotional disconnection. People worry constantly about their own body odor and whether others perceive them as unclean, sometimes showering excessively or overusing perfume. More intimately, many describe losing a sense of emotional closeness with partners and family members because they can no longer smell them. One participant in a qualitative study described the experience of anosmia as being “behind a glass” in relationships, feeling present but separated by an invisible barrier.19PLoS ONE. Experiences of living without a sense of smell: Like “Being Behind Glass” Others described the unique scent of a partner or child as something that had provided a feeling of security and connection, now gone.
Anosmia is also an invisible condition, which creates its own social friction. Because people cannot see it, they tend to either dismiss it as trivial or react with dramatic disbelief, and neither response feels supportive. The result is that many people with anosmia simply stop talking about it, which deepens the isolation.
How Smell Is Tested Clinically
Self-reported smell loss is unreliable. People tend to overestimate or underestimate their own olfactory ability, so doctors use standardized tests. One of the most widely used in clinical and research settings is the Sniffin’ Sticks test, developed in the late 1990s and validated across multiple countries. It uses felt-tip pen-like devices filled with odors and evaluates three dimensions of smell: the threshold at which you first detect an odor, your ability to discriminate between different odors, and your ability to identify specific smells by name.20PubMed. Sniffin’Sticks: a new olfactory test battery All three subtests reliably distinguish between people with normal smell and those with anosmia.21PubMed. How to assess olfactory performance with the Sniffin’ Sticks test
In the United States, the University of Pennsylvania Smell Identification Test (UPSIT), a scratch-and-sniff booklet, is more commonly used. Both tests serve the same purpose: giving clinicians an objective measure of how well the olfactory system is working, which in turn helps guide diagnosis and track whether a treatment is making a difference.
Olfactory Training
For people whose smell loss is not caused by an ongoing physical blockage, the most widely recommended treatment is olfactory training. This involves deliberately and attentively sniffing a set of strong, distinct odors, usually four, twice a day for at least three to four months. The classic protocol uses rose, lemon, clove, and eucalyptus. The idea is to stimulate the regeneration and reconnection of olfactory neurons through repeated, focused exposure.
Olfactory training is the most commonly used clinical method for post-viral and post-traumatic smell loss, though the degree of improvement varies widely between individuals.22Springer. Potential role of terpenes in recovery from olfactory dysfunction with olfactory training: a review Some researchers are exploring whether plant-derived compounds called terpenes, which have anti-inflammatory and potentially neuroprotective properties, could enhance the effectiveness of olfactory training. The evidence is still early, but the biological rationale is plausible since terpenes interact with some of the same receptor and signaling systems used by the olfactory epithelium.
Platelet-Rich Plasma and Experimental Approaches
One of the more promising experimental treatments for persistent smell loss involves injecting platelet-rich plasma, or PRP, directly into the nasal tissue near the olfactory epithelium. PRP is derived from your own blood and concentrated with growth factors that promote tissue repair and reduce inflammation. A recent systematic review and meta-analysis of controlled studies found a moderate, significant improvement in objective smell scores for patients who received PRP compared to controls. The average improvement exceeded the threshold considered clinically meaningful, and benefits were seen across post-COVID, other post-viral, and post-traumatic causes of smell loss. Side effects were mostly limited to mild, short-lived nosebleeds.23PubMed. Navigating the efficacy, safety and outcome of intranasal platelet-rich plasma for persistent olfactory loss: a systematic review and meta-analysis
Earlier smaller studies had shown encouraging results as well, with patients improving after one or two PRP injections and no serious adverse outcomes.24PubMed Central. Platelet-rich Plasma in Management of Anosmia (Single Versus Double Injections) Stem cell transplantation is another line of active investigation, exploiting the fact that the olfactory epithelium is one of the few places in the body where neurons naturally regenerate throughout life.25PubMed Central. Novel Therapies in Olfactory Disorders These approaches are still considered experimental, but they represent a meaningful shift from the previous era in which doctors essentially had nothing to offer patients with persistent neurosensory smell loss.
Why Humans Already Have a Weakened Sense of Smell
It is worth knowing that even at baseline, human smell ability is diminished compared to most mammals. Over 60% of human olfactory receptor genes have accumulated mutations that render them non-functional, compared to roughly 20% in mice. Humans appear to have lost functional olfactory receptor genes about four times faster than other species studied, likely because the evolutionary pressure to maintain a powerful sense of smell relaxed as our ancestors came to rely more on vision and social cognition.26PubMed Central. Human specific loss of olfactory receptor genes Even among non-human primates, the fraction of olfactory pseudogenes is lower than in humans, suggesting this is a specifically human trajectory.
This evolutionary context helps explain why smell loss can go unnoticed for so long. We are already working with a reduced toolkit, so gradual further decline, whether from aging, medication, or early neurodegeneration, can fly under the radar until it becomes severe. It also underscores why formal testing, rather than self-assessment, matters when smell loss is suspected.