When Familiar Scents Smell Wrong: Understanding and Managing Parosmia

Parosmia: Pathophysiology and Management

Parosmia is a smell disorder where people experience smells differently than they should. Instead of just not being able to smell things, like in some other smell disorders, those with parosmia might find that familiar scents suddenly smell distorted or unpleasant. This happens when the brain mismatches its memory of a smell and what it’s actually sensing.

Prevalence and COVID-19 Impact

Parosmia is less common than some other smell disorders, affecting about 2.1% to 4.8% of the general population. However, among those already experiencing smell problems, the rate is higher, particularly in specialist medical centres where 18% to 34% of patients report parosmia. The COVID-19 pandemic increased cases of parosmia because loss of smell was a key symptom of the virus. Studies show that between 32% and 75% of COVID-19 patients developedparosmia, often emerging one to three months after initial smell loss

Covid-related arosmia is more common in younger people, particularly women, who retain some ability to smell (meaning they have a reduced sense of smell but not complete loss). This contrasts with pre-pandemic cases, which were primarily seen in post-menopausal women.

Key Takeaway: Parosmia is relatively rare but became more common after COVID-19, affecting up to 75% of those who lost their sense of smell. Younger people, especially women, are more likely to experience it.

What Causes Parosmia?

Most people with parosmia have reduced smell function (called hyposmia), rather than complete loss of smell (anosmia), and less than 10% of patients have normal smell function (normosmia). The most common causes of parosmia are viral infections (such as colds, flu, or Covid-19) and head injuries. Less common causes include chronic sinus conditions and, rarely, medications such as certain antibiotics, antidepressants, chemotherapy drugs, and diabetes medications, but there’s no strong evidence to suggest that medication is a major cause.

People who develop parosmia after having COVID-19 often experience a much lower quality of life compared to those without parosmia, even if their sense of smell has partially returned. Since many common triggers are food smells, parosmia can lead to changes in eating habits, cooking, and social interactions. The mix of food and waste-like odours can add to the discomfort. Some of the effects include weight loss, anxiety, social isolation, depression, changes in intimacy, and, in some cases, increased alcohol consumption as a coping mechanism.

Key Takeaway: Most cases of parosmia follow viral infections like COVID-19 or result from head injuries. While medications have been considered as a cause, there’s no strong evidence linking them to parosmia.

How Smell Normally Works

Once an odour molecule touches an olfactory receptor, , signals are sent through nerve fibres to the brain’s olfactory bulb, which processes the smell and sends it to different areas of the brain for further processing. The brain then interprets and categorises the odour, linking it to memory and emotions of that smell, linking it with memories and emotions.

The brain forms this mental picture of a smell based on patterns of neural activity in the brain. Each olfactory neuron responds to just one type of odour, helping the brain identify different smells.

Each olfactory sensory neuron (OSN) in the nose is specialised to detect only one type of odour, thanks to a single olfactory receptor gene. This means that each neuron responds to a specific set of smells. When different odour molecules are detected, the brain interprets them by combining signals from different olfactory receptors.The brain then processes the smell information in different areas:

The amygdala creates emotional responses to odours (like whether we find something pleasant or unpleasant).
The orbitofrontal cortex is involved in identifying odors and linking them to meaning.
The insula helps with understanding the quality of smells and combining smell with taste.
The hippocampus helps with remembering odours and distinguishing different smells.

Smell is a complex and multi-layered process, but the brain is incredibly good at recognising odours with great accuracy. This precise system, however, seems to break down in parosmia, leading to distorted or unpleasant smells.

Key Takeaway: Smell is processed through a complex system involving the nose, olfactory bulb, and brain. Parosmia occurs when this system malfunctions, leading to distorted smell perceptions.

The Peripheral Hypothesis, or ‘miswiring theory’

One of the most common theories for parosmia, dating back to 1918, is the “miswiring” theory. This theory suggests that after an injury (like a viral infection), damage to the olfactory sensory neurons (OSNs) or supporting cells can cause problems with how odours are processed. When OSNs are damaged or not functioning properly, the brain might get an incomplete or “off-key” version of the smell, leading to distorted perceptions.

When these neurons regenerate, they may connect incorrectly, sending distorted signals to the brain. Imaging studies show differences in olfactory bulb size and brain activity in people with parosmia, suggesting that both peripheral nerve damage and central processing issues contribute.

In one study that looked at how the olfactory system works in people with parosmia after a viral infection, researchers used a special imaging technique to trace the olfactory pathway in the nose and brain. They found that people with parosmia who still had some sense of smell (normosmic) had higher levels of a marker in the nasal cavity than those with reduced smell (hyposmic). However, the way the marker moved from the nose to the brain didn’t differ much between the two groups. The study also showed that people with parosmia (whether they had normal or reduced smell) had larger olfactory bulbs compared to healthy individuals, but their bulbs were still smaller when compared to the general population.

One theory suggests that during the recovery of OSNs, the brain might become overly sensitive to certain types of smells, particularly strong, pungent ones, which leads to distorted perceptions of some odours but not others. A study found that specific molecules in coffee triggered parosmia in affected patients but didn’t cause distortions in people without parosmia. This points to the idea that certain receptors in the nose might be more responsible for parosmia than others.

Overall, the main idea behind these theories is that in parosmia, the brain’s system for processing smells is disrupted. This can happen right after an injury, or later on as the olfactory system tries to regenerate. The disruption could involve damaged neurons, misconnected receptors, or missing signals that help regulate smell, leading to the unusual smell experiences seen in parosmia. Some research also suggests that COVID-19 may affect the nervous system in ways that contribute to parosmia, though this idea is still being explored.

Key Takeaway: Parosmia may be caused by damaged olfactory neurons that regenerate incorrectly, sending mixed signals to the brain. This can result in certain smells being perceived as unpleasant or distorted.

The Central Hypothesis

The central hypothesis suggests that parosmia happens because of problems in how the brain processes smell signals. Specifically, the olfactory bulbs and other brain areas involved in interpreting smells might not be working correctly.

Research using brain imaging supports this idea. In one study using functional MRI, patients with just reduced smell (hyposmia) showed typical brain activation in response to smells, though at a lower level. However, people with both reduced smell and parosmia had stronger brain activity in areas like the thalamus and putamen, which are involved in processing sensory information. A case report of one patient showed that PET/CT scans revealed lower brain activity in certain smell-processing regions, but MRI scans showed that their olfactory bulb was still intact. This suggests that in parosmia, the problem might be more about how the brain processes smell signals rather than just damage to the olfactory bulb.

Some studies have also looked at changes in the brain’s network connections. Hyposmic patients with parosmia had different brain connectivity compared to those who only had reduced smell, hinting that parosmia could be a functional issue in the brain.

Another idea is that parosmia might be caused by “short-circuiting” in neurons (brain cells) that are not properly insulated, leading to mixed-up signals. This theory is supported by some people with parosmia experiencing improvement after taking anti-seizure drugs like sodium valproate, pregabalin, or gabapentin, although these are not standard treatments for parosmia, and this class of drugs have side effects that may rule them out for some patients.

Key takeaway: Brain imaging studies suggest parosmia might be linked to how the brain processes smell signals, not just damage in the nose. Changes in brain activity could play a role in distorted smell perceptions.

Management of Parosmia

Since there is no definitive cure, treatment focuses on symptom management and gradual recovery. One challenge in treating parosmia is that most studies on treatment effectiveness focus on how much people can smell, not on how well they handle distorted smells. This makes it hard to know if the treatments are truly helping with parosmia. Also, the way parosmia is measured in different studies varies, which makes it difficult to compare results across research.

Coping Strategies

Since many triggers for parosmia are food-related, patients often need guidance on how to deal with food. This support can be helpful for nutrition and improving quality of life. The natural response might be to avoid foods that trigger parosmia, but this can lead to malnutrition, especially if those foods are important protein sources like meat and eggs. Avoiding certain foods completely might also slow down the adjustment process.

Some strategies that may help include eating the triggering foods chilled or at room temperature, as this can reduce the strong odours released during cooking. Cooking methods like boiling or steaming might be easier to tolerate compared to roasting or frying, which can release stronger smells. Since parosmia affects people differently, finding the most comfortable way to prepare food might involve some trial and error.

Smell Training

Smell training is recommended for people who have lost their sense of smell, especially after infections or injuries. It involves sniffing four different odours—usually lemon, rose, eucalyptus, and clove—twice a day for at least 15 seconds each. Some variations of OT may use different odours or numbers of smells. Studies have shown that olfactory training can help improve the sense of smell in people who have lost it after viral infections. While this may not directly fix parosmia, it could help improve overall smell function over time.

One study on 153 people with post-infectious parosmia found that after about 6 months of smell training, participants showed significant improvement in their ability to discriminate and identify odors. Another study on 75 people with post-COVID parosmia showed that smell training helped improve their sense of smell more quickly than a control group who didn’t, although improvements in parosmia itself took longer to appear—at about 6 to 9 months.

There’s also evidence that combining smell training with other treatments, like specific supplements, might help improve parosmia resolution. However, more research is needed to confirm these findings.

Sticking with smell training

A common issue with smell training is that people often don’t stick with it for long enough. A survey of 450 people with smell problems found that 76% stopped after less than 4 weeks, which is likely not enough time to see real improvements. It is most effective when done consistently for at least 6 weeks, and people whose quality of life is more impacted by parosmia are generally more motivated to continue. Healthcare providers can help by encouraging patients to stay committed to the practice, especially since it’s a safe and low-cost treatment.

Platelet-Rich Plasma (PRP)

Platelet-rich plasma (PRP) is made by taking a sample of a person’s own blood, spinning it in a centrifuge to concentrate the platelets (cells that help with healing), and then using that concentrated blood to promote healing in areas that need it. PRP has been used for a variety of medical purposes, like joint injuries, dentistry, and even hair restoration. Since COVID-related smell loss is thought to happen due to inflammation and damage to the olfactory system, PRP has been tested for this type of olfactory dysfunction with some success.

Two studies have tested PRP for parosmia specifically. In the first, 60 people with post-COVID parosmia were given either PRP injections or no PRP (but received other treatments like olfactory training and supplements). Both groups saw improvement in their parosmia, but the PRP group had a more significant improvement. However, their sense of smell wasn’t measured in this study.

In the second study, 159 patients with post-COVID olfactory issues participated. The PRP group (81 patients) received PRP injections and 3 months of smell training, while the control group (78 patients) only did smell training. After 10 weeks, the PRP group showed greater improvement in parosmia and overall smell function compared to the control group.

Gabapentin

Gabapentin is a medication typically used for nerve pain, but it has also been tested in treating parosmia. In a small case series of 12 patients with post-COVID parosmia, gabapentin was given alongside olfactory training and nasal steroid treatments. After 3 weeks of treatment, 6 out of 9 patients reported significant improvement in their parosmia. However, some patients experienced side effects like drowsiness, and two patients didn’t tolerate the medication.

In a larger, double-blind trial with 68 patients, gabapentin did not show a clear benefit over a placebo in improving smell or parosmia. While some patients on gabapentin reported less severe smell distortion, the improvement wasn’t significantly better than those on a placebo.

Other Approaches

Other treatments have been tested, but their results have been less clear. For example, studies on oral alpha-lipoic acid (ALA) and intranasal sodium citrate didn’t show significant benefits for parosmia. Some patients have tried intranasal vitamin A drops, which may help with general smell loss, but there’s no evidence yet that they specifically improve parosmia.

There’s also a case report of a person with parosmia who had a surgical procedure to block the right olfactory cleft (a part of the nose related to smell). The procedure successfully resolved the parosmia, and the improvement lasted for two years.

In summary, while treatments like PRP and gabapentin show some promise for improving parosmia, more research is needed to fully understand their effectiveness. Other approaches, like certain supplements or even surgery, have been explored but with limited evidence so far.

A Note on Corticosteroids

Corticosteroids are commonly prescribed for olfactory issues, including parosmia, especially after COVID-19. This is likely because they are known for their anti-inflammatory effects and are used for conditions like allergies and sinus problems. In a survey of 209 patients with post-COVID parosmia, almost half had used nasal corticosteroid sprays. However, there haven’t been any studies specifically designed to test corticosteroids on parosmia itself. Most of the research looks at their effects on general smell loss.

A review of studies on COVID-related smell problems found no strong evidence that nasal corticosteroids help recover smell or improve parosmia. Because of this, there is no clear support for using corticosteroids as a routine treatment for parosmia. Doctors should also be careful when prescribing corticosteroids, especially in systemic forms, due to potential risks and the lack of proven benefit for parosmia.

Key takeaway: There’s no single cure for parosmia, but symptom management strategies can help. Since measuring progress is challenging, treatment focuses on practical solutions and improving quality of life.

Conclusion

Parosmia, where familiar smells turn into strange or unpleasant odours, can be a challenging condition. It often improves over time, though recovery can take months or even years. While the exact causes of parosmia are still not fully understood, recent research points to both peripheral (in the nose) and central (in the brain) factors. Currently, treatment options for parosmia are limited, and more research with larger clinical trials is needed to better understand how to effectively manage and treat it.

Sources:

Xinni Xu1,2 · Jerry Hadi Juratli1,3 · Basile Nicolas Landis4 · Thomas Hummel1

Current Allergy and Asthma Reports (2025) 25:10