The loss of smell (anosmia) or a reduction in smell (hyposmia) emerged as a distinctive and highly prevalent symptom of COVID-19 infection. This sensory loss can be profoundly distressing for patients. While the symptom is often temporary, the timeline for a return to normal function is highly variable among individuals. Understanding typical recovery patterns is important for setting expectations, though the exact duration remains unpredictable.
How COVID-19 Affects the Sense of Smell
The mechanism by which SARS-CoV-2 causes smell loss is distinct from that of a common cold. The virus does not directly infect the olfactory neurons, the nerve cells responsible for detecting odors. Instead, the virus targets the supporting cells (sustentacular cells) within the olfactory epithelium, the tissue lining the upper nasal cavity. These cells express the ACE2 receptor and the TMPRSS2 enzyme, which the virus uses to gain entry.
Infection and damage to these supporting cells cause a temporary disruption in the function of neighboring olfactory neurons. Since sustentacular cells provide metabolic and structural support, their injury leads to neural dysfunction and inflammation. This explains why smell loss can occur suddenly, even without the nasal congestion typical of other viral infections. Because the neurons are largely spared, the system retains the ability to regenerate and recover function once the damage resolves.
Defining Olfactory Recovery Timelines
For the majority of patients, the sense of smell returns relatively quickly, often within the first few weeks after the acute infection. Studies indicate the median recovery time for those with a swift return of function is approximately 11.5 days. Up to 88% of individuals report a complete or partial recovery within two months.
Recovery continues past this early period for those who do not see an immediate return. An estimated 95% of patients recover olfactory function within six months following the initial infection. However, a percentage of people face persistent olfactory dysfunction lasting six months or longer.
Research tracking patients for up to two years found that approximately 90% of those with mild COVID-19 fully regain their sense of smell, suggesting gradual improvement occurs over a prolonged period. Factors associated with slower recovery include more severe smell loss, being female, and older age. This prolonged recovery is based on the slow process of neural repair and regeneration.
Strategies to Encourage Olfactory Recovery
For individuals experiencing persistent smell loss, olfactory training is recommended to encourage recovery. This non-invasive method involves actively sniffing a series of specific scents every day. The practice aims to stimulate and re-establish neural pathways between the nasal cavity and the brain.
The standard protocol involves using four distinct scent categories: floral, fruity, spicy, and resinous. Common examples include rose, lemon, clove, and eucalyptus essential oils. Training should be performed at least twice daily, with each scent sniffed for 15 to 20 seconds while concentrating on the memory of the odor.
Training should be continued for a minimum of 12 weeks. Rotating to a new set of four scents after three months may offer additional benefit. Olfactory training is considered the primary, most effective self-directed intervention, though nasal steroid rinses may be useful if inflammation is present. Improvements can take many months to manifest.
Managing Distorted or Persistent Smell Loss
As the olfactory system recovers, some individuals may develop qualitative disorders rather than a simple loss of function. Parosmia is one such condition, characterized by a distorted sense of smell where normal odors are perceived as unpleasant, often described as burnt, rotten, or chemical. Another symptom is phantosmia, which involves perceiving phantom smells that are not actually present.
These qualitative changes can significantly impact a person’s quality of life and may appear after a period of initial recovery. If smell function has not returned or has become severely distorted after six to nine months, consulting a specialist, such as an Ear, Nose, and Throat (ENT) doctor or a neurologist, is advisable. Specialists can assess the severity of the dysfunction and discuss treatment options for long-term, persistent symptoms.