The loss of smell, known as anosmia, became one of the most recognized symptoms of COVID-19, often appearing suddenly without the typical nasal congestion of a common cold. This inability to detect scents, or sometimes a distorted sense of smell called parosmia, signaled a neurological disruption rather than a simple blockage. A significant percentage of people infected with SARS-CoV-2 experienced some form of olfactory dysfunction. Unlike the temporary smell loss seen with a typical head cold, which is caused by physical obstruction, the anosmia resulting from COVID-19 involves damage at the cellular level within the olfactory system.
How Viral Infection Impacts the Olfactory System
The virus that causes COVID-19 does not primarily target the olfactory neurons, the nerve cells responsible for detecting odors. Instead, the virus infects the supporting cells within the nasal lining, specifically the sustentacular cells of the olfactory epithelium. These supporting cells are rich in the ACE2 receptor and TMPRSS2 enzyme that the SARS-CoV-2 virus uses to enter human cells.
Infection of these sustentacular cells triggers a powerful inflammatory response in the olfactory epithelium. This localized inflammation and damage disrupt the entire olfactory system’s function, including the olfactory sensory neurons. The neurons themselves suffer damage to their cilia, hairlike structures that contain odor receptors, limiting their ability to transmit scent information to the brain.
Understanding the Recovery Timeline
Recovery from post-COVID smell loss is highly variable, ranging from a quick return in a few days to a process that can take many months or even years. For many, the sense of smell begins to return rapidly within the first few weeks after the acute infection has passed. A large majority of patients, over 80%, report a full or near-full recovery within four to six months.
However, the recovery process often involves qualitative changes to smell, most commonly parosmia, which is the perception of distorted or unpleasant odors. Parosmia frequently emerges during the regeneration phase as the damaged olfactory neurons begin to rewire and recover. Its appearance is generally seen as a sign that the system is attempting to repair itself.
The speed of recovery is influenced by factors including the severity of the initial smell loss and the individual’s age. Younger people and those who experienced a milder initial olfactory dysfunction tend to recover faster and more completely. Recovery can continue for up to two years, with nearly 90% of those with mild symptoms regaining their senses within that timeframe.
Active Steps for Olfactory Rehabilitation
One of the most effective methods for encouraging the return of smell is olfactory training, or smell training. This technique works by stimulating and encouraging the regeneration and rewiring of the damaged neural pathways. The standard protocol involves repeated, mindful exposure to a set of distinct, strong scents.
The recommended routine typically uses four distinct essential oils, representing different odor categories:
- Floral (such as rose)
- Fruity (lemon or citrus)
- Spicy (clove)
- Resinous (eucalyptus)
The practice involves sniffing each of the four scents for about 15 to 20 seconds, one after the other, twice a day. It is important to focus on the scent and try to recall what the odor was like before the illness, which helps engage the brain’s memory centers.
Consistency is paramount with this training, as results are often gradual and may take several months to become noticeable. After three months, many specialists suggest switching to a new set of four distinct scents to broaden the stimulation to different odor receptors and continue rehabilitation. This training addresses the underlying neurological damage, not physical congestion.
When Smell Changes Become Chronic
Smell loss is considered chronic when there has been no significant improvement six to twelve months following the initial infection. While the majority of people recover spontaneously, those with significant dysfunction after two months should seek professional guidance. Chronic smell loss can negatively impact quality of life, leading to issues like weight changes, depression, and the inability to detect environmental dangers such as gas leaks or spoiled food.
Consulting with a specialist, such as an Ear, Nose, and Throat (ENT) doctor or a neurologist, is advisable to rule out other possible causes for the olfactory dysfunction, like nasal polyps or inflammatory issues. A medical professional may discuss options such as prescription nasal steroid rinses, which can help reduce lingering inflammation in the olfactory area. Although there are currently no FDA-approved drugs specifically for post-viral smell loss, a specialist can offer objective testing and a long-term management plan.