For the many Covid-19 patients who have lost all or some of their sense of smell, doctors recommend an olfactory version of physical therapy called "smell training." Writing for the New York Times, Christina Caron talks with multiple experts to demystify the (surprisingly tedious) process.

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What is smell training?

According to Caron, a large proportion of people who contract Covid-19 report losing their sense of smell—entirely or in part—or experiencing parosmia, a disorder that "causes previously normal odors to develop a new, often unpleasant aroma." In fact, at least one meta-analysis, published in September 2020, found that up to 77% of Covid-19 patients report some form of smell loss, she writes.

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Experts are still unclear how Covid-19 causes smell loss. However, according to Caron, experts believe that parosmia "occurs because the neural pathways from the nose to the brain have been disrupted." According to Pamela Dalton, a faculty member at the Monell Chemical Senses Center, it's "kind of like a telephone operator from the 1950s connecting the wrong party to another line."

Most people who've lost their sense of smell because of Covid-19 regain it within weeks or months of being infected. However, for those who don't regain their sense of smell in a couple weeks, experts generally recommend smell training—the olfactory equivalent of physical therapy, Caron writes.

According to Caron, the process involves smelling a variety of potent scents twice a day—for weeks or months at a time—in an effort "to stimulate and restore the olfactory system," or at least "to help it function better."

But according to Chrissi Kelly, a member of the Global Consortium for Chemosensory Research and founder of AbScent, smell training—much like physical therapy—is "not a quick fix. You have to keep up with it."

Does smell training work?

"Robust studies examining the efficacy of olfactory training among Covid survivors have not yet been published," Caron writes, in part because many patients suffering from Covid-related smell loss recover within a few weeks of infection.

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But a few studies have begun to examine the issue. For instance, one study published in January looked at patients from 18 hospitals in Europe and found that the majority of 1,363 patients who experienced olfactory dysfunction—and who were encouraged to do smell training at home—saw their sense of smell recover within two months, and 40% saw it recover within two weeks. Jerome Lechien, a professor of otolaryngology at the University Hospital of Brussels and one of the study's authors, said while it's not clear how many patients did the training, 95% had recovered their sense of smell six months after the onset of their symptoms.

However, several studies "have demonstrated that smell training can help people who have lost some or all of their senses of smell to other viral illnesses like sinus infections," Caron writes, which is "why it is widely considered the best option for those who can no longer smell properly after contracting Covid." Perhaps most importantly, the therapy "has no risk," Dalton added—"except boredom."

Nonetheless, before starting smell training, experts say it's a good idea to make sure other conditions aren't affecting your sense of smell, Caron reports.

Sunthosh Sivam, an ear, nose, and throat specialist and assistant professor at the Baylor College of Medicine, said he saw a patient who "had smell dysfunction following Covid-19, and it turned out they had inflammatory nasal polyps." Once those polyps were removed, the patient's sense of smell improved.

How does smell training work?

According to Caron, there's no uniform method of smell training, but the experts she spoke with tended to offer similar advice.

First, experts recommended selecting four scents "that are familiar to you and that evoke strong memories," Caron writes. A home chef may select spices from his or her pantry, for instance, while someone else may choose a favorite cologne. People have also reported success by purchasing a smell kit containing essential oils—such as rose, eucalyptus, and lemon—or, according to Dalton, even using items that smell bad, such as spoiled milk.

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Then, experts recommend you keep the scents in a location that's easy to access, such as a bedside table, and smell each scent for around 20 seconds, taking short "'bunny sniffs'" rather than deep inhalations, Caron reports.

It may also help to look at a picture of whatever scent you're inhaling, Nicholas Rowan, an assistant professor of otolaryngology-head and neck surgery at the Johns Hopkins University School of Medicine, said.

People should also try to imagine what that the item "used to smell or taste like," Caron writes. "It's not simply the act of smelling something, but it's also this sort of mindful imagining of what that smelled like when you were eating it or when you put it on your skin—if it was a lotion, for example," she said. "It just makes it more enjoyable to continue with the process."

How long does smell training take?

According to Caron, people should expect to maintain this routine for quite some time. In general, experts recommend doing smell training twice a day for three months—but you should be prepared to "keep on training for a year if you have to," Thomas Hummel, a researcher at the Smell and Taste Clinic at the Technical University of Dresden, said.

Progress may be slow, Hummel said, and it can be more difficult for older people, who have fewer olfactory receptor neurons. As a result, it can be difficult for patients to stick with the training.

"It's very frustrating for patients," Rowan said. "They seek out this care because they can't smell and want it fixed and then we say, 'Hey, use this sensory function that you don't have.'"

However, Rowan added that smell training "is the best thing out there" (Caron, New York Times, 3/26).