Olfactory training is the most studied and best-supported rehabilitation method for smell loss caused by COVID-19. The technique involves deliberately sniffing a small set of strong odors for about 20 seconds each, twice a day, over weeks to months. A 2024 systematic review of the evidence concluded that this practice alone produces meaningful improvements in chronic smell problems after COVID, and one controlled trial found that 40 percent of patients who trained for 12 weeks improved past a clinically important threshold, compared to just 6 percent of untreated controls. The approach is low-cost, safe, and something you can start at home, but the details of how to do it well, how long it takes, and who benefits most are worth understanding.
Why COVID Takes Away Your Sense of Smell
To understand why repeated sniffing of essential oils can help, it helps to know what the virus actually damages. SARS-CoV-2 does not appear to infect the smell-detecting nerve cells (olfactory sensory neurons) themselves. Instead, it attacks the support cells surrounding them, called sustentacular cells. These cells maintain the chemical environment the neurons need to function, and multiple studies in both human tissue and animal models have confirmed they are the primary viral target in the nose’s smell-sensing lining.1PubMed Central. SARS-CoV-2 infection of sustentacular cells disrupts olfactory signaling pathways2Cell. SARS-CoV-2 infects sustentacular cells of the olfactory mucosa in COVID-19 One research group put it plainly: the sensory neurons become affected without ever getting infected, because the cells that support them are knocked out of commission.
In most people, this disruption is temporary. The support cells recover, the sensory neurons resume functioning, and smell comes back within a few weeks. But for a significant minority, the damage lingers. Biopsies from people with long-lasting smell loss after COVID have shown ongoing inflammatory signaling in the olfactory lining even after the virus itself is undetectable, along with a reduced number of olfactory neurons relative to support cells.3Science Translational Medicine. Persistent post–COVID-19 smell loss is associated with immune cell infiltration and altered gene expression in olfactory epithelium In other words, the immune response to the initial infection keeps simmering, and the nerve cells that detect odors become collateral damage in a self-sustaining cycle of inflammation. That is the problem olfactory training is trying to break through.
What Olfactory Training Actually Involves
The standard protocol, first developed in 2009, is straightforward. You choose four distinct scents, classically rose, lemon, clove, and eucalyptus, each representing a different odor category. You open each jar and hold it close to your nose, sniffing gently and intentionally for about 20 seconds while trying to recall what the scent should smell like. Then you move to the next one. You repeat the full set twice a day, morning and evening, for at least 12 weeks.4PubMed Central. Post-viral olfactory loss and parosmia
Most people use essential oils dabbed onto cotton pads inside small glass jars, though commercially available kits now package them more conveniently. The specific scents are not set in stone. What matters is choosing odors that are potent, represent different odor families, and ideally have some personal familiarity. The mental effort of actively thinking about what you are smelling while you sniff appears to be part of what makes it work, not just passive exposure.
How Sniffing Retrains the Brain
The benefit of olfactory training seems to operate on two levels. At the peripheral level, in the nose, repeated stimulation of the olfactory lining may encourage the regeneration of damaged sensory neurons. Animal studies have found that olfactory training speeds up the recovery of olfactory nerve tissue after injury.5PubMed Central. The neuroplastic effect of olfactory training to the recovery of olfactory system in mouse model But researchers increasingly suspect the bigger effect is central, happening in the brain itself. Neuroimaging studies have found that olfactory training changes how brain regions involved in smell processing connect to one another, essentially reorganizing the neural wiring.6PubMed Central. Recovery of olfactory function induces neuroplasticity effects in patients with smell loss
A systematic review of the neuroimaging literature confirmed that olfactory training is associated with increases in the size of smell-related brain structures, including the olfactory bulb (the brain’s first relay station for scent information) and the hippocampus.7PubMed Central. Does Olfactory Training Improve Brain Function and Cognition? A Systematic Review That is a remarkable finding: a simple behavioral exercise performed at home can physically change the volume of brain tissue. The idea is that by repeatedly activating the smell pathway, even when little or no scent is perceived initially, you are providing the neural circuits with a training signal that helps them rebuild and reconnect over time.
The Clinical Evidence for Effectiveness
Multiple trials have now tested olfactory training specifically in people who lost their sense of smell after COVID. A 2024 systematic review pooling the available studies concluded that olfactory training alone produces significant improvements in chronic smell dysfunction following the infection.8PubMed Central. The effectiveness of olfactory training for chronic olfactory disorder following COVID-19: a systematic review The improvements are most clearly seen in odor sensitivity, the ability to detect faint smells, rather than in odor identification or discrimination, suggesting the training’s greatest impact is on the threshold at which you can perceive a scent at all.
One of the stronger individual trials randomized post-COVID patients with persistent smell loss to either 12 weeks of classical olfactory training or no treatment. Forty percent of the training group crossed the threshold for a clinically meaningful improvement in their smell scores, compared with six percent of the untreated control group.9PubMed Central. Effects of classical olfactory training in patients with COVID-19-related persistent loss of smell Those are encouraging numbers, though they also underscore that the training does not work for everyone. Most participants improved somewhat, but roughly six in ten did not reach the bar for a clinically meaningful change even with consistent training.
One caveat worth keeping in mind: a prospective study of patients with severe, long-term post-COVID smell loss found only limited objective improvement from training, and noted a gap between what patients reported subjectively and what standardized smell tests showed.10European Archives of Oto-Rhino-Laryngology. Limited observed efficacy of olfactory training in severe long-term post-COVID-19 syndrome: a prospective cohort study The people who had been without smell the longest and had the most severe dysfunction were the hardest to help. That does not mean training is useless in those cases, but expectations should be realistic: improvement tends to be gradual and partial, not a dramatic on-off switch.
Does Starting Earlier Make a Difference?
Timing matters. A meta-analysis comparing patients who began olfactory training during the acute phase (within the first month of smell loss) versus those who started later found that the acute group improved substantially more.11American Journal of Rhinology & Allergy. The Efficacy of Olfactory Training as a Treatment for Olfactory Disorders Caused by Coronavirus Disease-2019: A Systematic Review and Meta-Analysis Both groups showed clinically significant gains, but the effect size for early starters was more than double that of the late starters. This makes biological sense: the earlier you begin providing regular stimulation to an olfactory system that is trying to recover, the more you can take advantage of the window when neurons are actively regenerating and neural circuits are most plastic.
The practical takeaway is simple: if your smell has not come back within a couple of weeks after a COVID infection, do not wait months to see whether it resolves on its own before starting to train. You can begin while still in the early recovery period and potentially capture the greatest window of benefit. There is no downside to starting early, and the evidence tilts toward better outcomes.
Training for Distorted Smells
Not all post-COVID smell problems involve total or partial loss. Many people develop parosmia, a condition where previously normal smells become distorted, often in deeply unpleasant ways. Coffee smells like sewage, onions smell like rotting meat, your partner’s skin smells chemical and wrong. Parosmia is thought to arise when olfactory neurons regenerate but wire up incorrectly, sending scrambled signals to the brain. It is, paradoxically, a sign of recovery, but an agonizing one.
Olfactory training appears to help parosmia as well. A study of patients with post-COVID parosmia found that a modified training protocol was effective in reducing distortions.12PubMed Central. Modified Olfactory Training Is an Effective Treatment Method for COVID‐19 Induced Parosmia Interestingly, evidence suggests that patients with parosmia may actually benefit more from training than those with simple quantitative loss. A study of over 150 patients with post-infectious smell dysfunction found that people who had parosmia were more likely to achieve meaningful improvement in their ability to identify and distinguish odors compared to those who only had reduced smell.13PubMed Central. Post-viral olfactory loss and parosmia The hypothesis is that parosmia reflects miswired but active neural circuits, which the brain can learn to reinterpret and correct through the repeated, structured exposure that training provides.
Beyond the Classic Four Scents
The original rose-lemon-clove-eucalyptus protocol remains the most widely studied, but researchers have been testing whether modified versions work better. One approach called “modified olfactory training” swaps in new scents every 12 weeks to keep the stimulation novel and prevent habituation. Another, more recent innovation involves terpene-centered training, which uses groups of chemically related odors delivered both through the nose (sniffing) and through the mouth (as flavor during eating). A trial comparing this dual-route terpene approach against both the classical protocol and modified training found that the terpene group showed significantly greater improvements in overall smell scores and odor identification at 9 and 12 months, with the greatest reductions in parosmia severity as well.14The Laryngoscope. Terpene‐Centered Olfactory Training in Postinfectious Olfactory Dysfunction: A Dual‐Route Approach
Why would training through both routes matter? When you eat food, volatile compounds travel from the back of your throat up to the olfactory receptors in a process called retronasal olfaction, which is distinct from the orthonasal route used when you sniff something directly. Training both pathways may engage a broader network of neural connections, providing a richer training signal. For people whose most distressing symptom is that food tastes wrong, exercising the retronasal route directly addresses the problem where it is experienced.
Technology is also changing how people access and stick with training. A randomized trial tested a web-based application that guided patients through a bimodal training protocol combining olfactory and visual cues. The app-guided group showed improvements in both psychophysical test scores and subjective quality of life, and the structured digital format improved adherence compared to simply handing someone a set of jars and instructions.15PubMed. Web-application guided bimodal olfactory training for COVID-19 patients: a randomized trial
Why Compliance Is the Biggest Predictor of Success
The unsexy truth about olfactory training is that the people who benefit most are the people who actually do it consistently. Sniffing four jars twice a day sounds easy, but maintaining that routine for three months or longer is tedious, especially when you cannot smell anything at all and the exercise feels pointless. Dropout and inconsistency are the biggest practical obstacles.
A study of long-COVID patients with persistent smell loss found that full compliance with the training protocol was significantly associated with achieving meaningful improvement, with an odds ratio of about 8.16Danish Medical Journal. Olfactory training in long COVID-19 patients with lasting symptoms including olfactory dysfunction In practical terms, patients who stuck with the full program were roughly eight times more likely to reach a clinically meaningful improvement than those who trained inconsistently or stopped early. That is a large effect, and it suggests that many negative outcomes attributed to the training itself may actually be failures of adherence.
Anything that makes the routine easier to maintain is likely to improve results. Setting a consistent time (right after brushing your teeth in the morning and before bed), keeping the jars visible in a designated spot, using an app or phone reminder, and switching to new scents every few months to maintain novelty and motivation all help. Some clinicians now recommend the web-guided or multimodal approaches mentioned above specifically because they keep patients engaged.
The Gap Between What You Feel and What Tests Show
One complicating feature of smell rehabilitation is that your subjective sense of how well you can smell does not always match what standardized tests reveal. Studies have consistently found a mismatch between self-reported smell ability and psychophysical test results in COVID patients.17PubMed Central. Olfactory dysfunction in COVID-19; Self-report or olfactory dysfunction test? Some people who believe their smell has fully recovered still score below normal on objective testing. Others feel their smell is terrible but perform surprisingly well when asked to identify or discriminate odors in a controlled setting.
This gap can go both ways during olfactory training. A study of severe, long-term post-COVID patients found transient improvements that patients noticed subjectively, but the objective testing did not always back them up.18European Archives of Oto-Rhino-Laryngology. Limited observed efficacy of olfactory training in severe long-term post-COVID-19 syndrome: a prospective cohort study This does not necessarily mean the subjective improvements are illusory. Smell is deeply emotional and contextual; the brain can become better at interpreting partial signals even if the raw detection hardware has not fully recovered. But it does mean that tracking progress purely by how you feel day to day can be misleading. If you have access to a clinician who can administer a standardized smell test, periodic objective assessment gives you a more reliable picture of where you stand.
Combination Therapies and Add-Ons
Olfactory training is often used alongside other treatments, though the evidence for most combinations is thin. Oral or nasal corticosteroids are sometimes prescribed for post-COVID smell loss in hopes of reducing the inflammatory component, but the evidence is mixed and routine steroid use for this purpose is not strongly supported. Omega-3 fatty acid supplements, which have anti-inflammatory and nerve-supportive properties, have been tested alongside olfactory training. In one trial, patients receiving omega-3 capsules for 12 weeks showed significant improvement in their smell threshold scores, while the olfactory-training-only control group did not reach significance on that particular measure, though the overall rates of full smell recovery were modest in both groups (about 28 percent in the omega-3 group versus 21 percent in the training-only group).19PLOS ONE. Effect of any form of steroids in comparison with that of other medications on the duration of olfactory dysfunction in patients with COVID-19: A systematic review of randomized trials and quasi-experimental studies
Adding visual imagery to scent training is another area of active research. The idea is that looking at a picture of a rose while sniffing the rose essential oil may recruit additional brain networks and strengthen the training effect. The web-based app trial mentioned earlier incorporated this visual-olfactory pairing and found positive results, though disentangling the effect of the visual cue from the effect of better adherence through app guidance is difficult.
What Smell Loss Does to Daily Life
The urgency behind this research comes from the toll that persistent smell dysfunction takes on people’s lives. Smell loss is far more disabling than most people who have not experienced it imagine. It disrupts eating, since the flavor of food depends heavily on retronasal olfaction, leading to appetite changes, weight loss or gain, and the loss of one of life’s most basic pleasures. It creates safety hazards: you cannot smell a gas leak, smoke, or spoiled food. And the psychological effects run deep, including social withdrawal, anxiety, and a kind of grief for a sensory world that has gone flat and colorless.20Nutrition Today. No Smell, No Taste—Dealing With a “Senseless” Phase of the Pandemic
People with parosmia often have it worse than those with complete loss, because the distortions are actively repulsive rather than simply absent. The smell of cooking food, coffee, or a loved one’s body can trigger nausea. Meals become something to endure. Some people lose significant weight because so many foods have become intolerable. Understanding this context explains why even a modest improvement from olfactory training can feel transformative. Going from zero smell to faint smell, or from distorted smell to less distorted smell, can meaningfully change someone’s relationship with food, their emotional state, and their sense of safety in their own home.
Practical Tips for Getting Started
If you want to try olfactory training, you do not need a prescription or a clinic visit to begin. Here is what the evidence supports:
- Choose four scents: Rose, lemon, clove, and eucalyptus are the classic set. Essential oils are the most practical delivery method. Dab a few drops onto a cotton pad inside a small glass jar with a lid.
- Sniff with focus: Hold each jar close to your nose and inhale gently for about 20 seconds. While sniffing, actively try to recall or imagine the scent. This mental engagement appears important.
- Twice daily: Do the full set of four scents in the morning and again in the evening.
- Commit to at least 12 weeks: Improvement is slow. Most studies use 12 weeks as the minimum training period, and some protocols run 6 to 12 months.
- Swap scents periodically: After about 12 weeks, consider rotating in new odors (coffee, vanilla, mint, cinnamon are common choices) to prevent habituation and keep the training stimulus fresh.
- Consider adding retronasal training: Deliberately paying attention to flavors while eating, chewing slowly, and focusing on what you perceive through your mouth may help engage the retronasal pathway.
The barriers are low and the risks are essentially zero. The worst that happens is you spend a few minutes a day sniffing jars and nothing changes. Given the compliance data showing an eightfold difference in outcomes between consistent and inconsistent trainers, building the habit matters more than optimizing which particular scents you use.21Danish Medical Journal. Olfactory training in long COVID-19 patients with lasting symptoms including olfactory dysfunction If you have been struggling with smell loss for months and have not tried structured training, the evidence strongly favors giving it a serious and sustained effort.