Mind and Body

Does smell training work for Covid-19 anosmia? Here's what scientists know

Covid-19 has caused 1.6 million Americans to lose their sense for longer than six months.

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As someone with a freakishly good sense of smell, it’s hard to imagine how disturbing it would be if one day it simply disappeared. Sure, I’ve lost it occasionally when I’ve been sick, but nothing a few decongestants couldn’t fix.

But that’s the reality for many who had Covid-19. For most people, their smell comes back over time. For others, their total loss of smell — called anosmia — hasn’t returned. According to a study published Thursday in the journal JAMA Otolaryngol Head Neck Surg. an estimated 1.6 million Americans have lost their sense of smell due to Covid-19 for longer than six months. Scientists aren’t sure if the sense will return.

Some who are afflicted are buoyed by anecdotes of people getting their sense of smell back through olfactory or “smell” training. According to AbScent, a UK charity dedicated to “driving change to end smell disorders,” smell training is quite simple: a person actively sniffs “the same four scents every day, spending around 20 seconds on each scent.” While AbScent sells its own smell kits, the charity stresses they’re easily made at home for no cost and provide instructions for doing so.

Meanwhile, high-profile success stories include a restaurant critic who lost and regained her smell and was featured on The New York Times’ podcast, The Daily — offering hope to those who are anxious for their smell to return.

Our ability to smell isn’t just something that makes our food taste better or brings us pleasure when flowers bloom after a fresh rain — it’s a tool that keeps us alive. According to a 2014 study, people with anosmia are twice as likely to experience a smell-related hazardous event compared to those with typical olfactory function. Studies link anosmia in people infected with Covid-19 to increased psychological distress. Having a sense of smell isn’t a novelty —losing it changes your life.

Could solving such a stubborn problem really be as simple as sniffing spices? What do we know about the science behind smell training? Here is its complicated history, and what it means for people moving forward.

What is the origin of smell training?

While 90 percent of Covid-19 patients who experience smell loss will regain the sense in four weeks, some lose this sense for much longer.

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The idea traces back to German psychologist Thomas Hummel who, in 2009, wanted to help patients with olfactory dysfunction regain their sense of smell. Hummel asked the patients to smell four essential oils (rose, lemon, clove, and eucalyptus) for ten seconds twice daily for 12 weeks.

The oils were chosen to represent specific types of scents (flowery, fruity, spicy, and resinous). In that initial study, Hummel found the patients who participated in the “training” had improved olfactory function compared to controls.

In 2015, a study found that adding a wider range of scents increased the efficacy of smell training than using only the original four.

What viruses can result in anosmia?

Prior to Covid-19, we knew that other coronaviruses, as well as influenza viruses, can cause olfactory dysfunction, including anosmia. Typically, experts say, there are two reasons for viral olfactory dysfunction:

  1. Snot. Or, to put it in medical terms, “Through local inflammation, the lining of the nasal passages becomes swollen, the mucosa is lined by a film of nasal discharge, thereby hindering the odorant molecules to reach the corresponding receptors and bind to them, resulting in reduced ability to smell.”

Further, when you’re all plugged up, those odorant molecules have a tough time even getting into your nasal cavity in the first place.

2. However, snot can’t be blamed for Covid-19-associated olfactory dysfunction, though. Researchers say evidence strongly suggests that SARS-CoV-2 infects “support cells,” that provide metabolic and structural support to olfactory neurons as well as some stem cells and blood vessels.

What does the research show about smell training for Covid-19?

There have been several studies looking at the efficacy of smell training for Covid-19-associated olfactory dysfunction, but they all come with one big caveat: in time, most people will recover their sense of smell. Given that, it’s difficult to concretely parse how much smell training is working over time compared to how much just time is aiding recovery.

With that caveat in mind, many studies have found that participants with olfactory dysfunction who engaged in smell training saw more improvement than those in a control group. The margins, however, tend to be slight.

Dr. Erin O'Brien of the Mayo Clinic explains olfactory retraining.

A 2015 review of 10 studies evaluating the efficacy of olfactory training found that patients receiving olfactory training “experienced a statistically significant improvement” compared to controls in the Threshold, Discrimination, Identification (TDI) score, which is used to evaluate different aspects of olfactory function. Specifically, olfactory function was improved in discrimination by a mean difference of 1.92 and identification by a mean difference of 1.61.

In an interview with the publication neo.life, Brian Rotenberg, an otolaryngologist (ear, nose, and throat specialist) at London Health Science Centre in Ontario, Canada explained the theory behind the therapy.

“Almost like nasal physiotherapy, you can re-teach the nose how to smell,” he said.

While smell training is far from a guarantee, there’s also not much to lose by trying it. You can make your own smell training kit for free or very cheaply and, unlike some other treatments for anosmia, there are no side effects.

What other treatments are recommended for loss of smell after Covid-19?

Some doctors are prescribing steroids for anosmia. A 2021 study looked at the efficacy of steroids and smell training in Covid-19-associated anosmia and found that the combination of steroids and smell training together were more effective than either therapy alone.

Additionally, a trial is underway looking at Vitamin A as a possible treatment. Still, many researchers believe that smell training will prove to be the more effective treatment.

Of the available treatments for anosmia, smell training is certainly the option with the least amount of risk, even if the likelihood of reward is hard to quantify.

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