The sudden loss of the sense of smell, medically known as anosmia, is a common symptom experienced after a viral illness, most notably with respiratory infections like COVID-19. This sensory loss significantly impacts daily life, affecting a person’s ability to detect danger, such as smoke or spoiled food, and diminishing the enjoyment of eating. While this loss is often temporary, the recovery timeline varies greatly among individuals. Understanding the biological process behind this loss and the subsequent regeneration of the olfactory system helps set realistic expectations for recovery.
How Viral Infections Cause Smell Loss
When respiratory viruses invade the body, they typically target the olfactory epithelium, the patch of tissue high inside the nose responsible for detecting odors. Unlike the common cold, where congestion physically blocks odor molecules, many viruses cause smell loss even when the nasal passages remain clear. This loss is often due to damage to the supporting cells within the olfactory epithelium, rather than the direct destruction of the olfactory neurons themselves.
These supporting cells, called sustentacular cells, are equipped with the necessary receptors for the virus to gain entry, making them the primary target of infection. Sustentacular cells are essential for maintaining the delicate environment needed for the olfactory neurons to function properly, including providing metabolic support and clearing away odorants. When the virus attacks and damages these support cells, it disrupts the olfactory system, leading to a temporary shutdown of the smell function.
The resulting inflammation and loss of support cells temporarily disable the sensory neurons, which send odor signals to the brain. Because the olfactory neurons themselves are often spared, they can begin to recover once the infection and inflammation subside. This mechanism explains why smell loss can be sudden and profound, yet often resolves relatively quickly for many people.
Typical Recovery Timelines for Smell
The duration of anosmia following a viral infection can range from days to many months, but for most people, recovery begins within a few weeks. For those who experience a swift return of smell, the median recovery time has been reported to be around 11 to 14 days after the onset of the viral symptoms. Research suggests that 75% to 90% of individuals see a substantial recovery of their olfactory function within the first one to three months.
Recovery is a gradual process that can continue for up to a year or even longer for some individuals. Factors such as the initial severity of the smell loss and the patient’s age can influence the speed of recovery. Older patients and those with a greater initial loss may face a protracted timeline for regaining their full sense of smell.
Even after six months, recovery is still possible, and studies tracking patients have shown improvement continuing for up to two years post-infection. The fact that the olfactory sensory neurons can regenerate supports eventual recovery. Persistent smell dysfunction, lasting beyond six months, occurs in a smaller percentage of cases.
When Smell Returns Differently (Parosmia and Phantosmia)
As the olfactory system begins to heal and its neurons regenerate, some people may experience qualitative distortions in their sense of smell. Parosmia is a distortion where familiar scents are perceived as unpleasant, often described as smelling like burnt garbage, chemicals, or rancid food. For instance, the smell of coffee might be distorted into a sickening odor.
This phenomenon is thought to occur because the newly regenerating olfactory neurons are making incorrect connections to the olfactory bulb in the brain. Instead of correctly interpreting a specific odorant, the miswired connection sends a distorted signal. Phantosmia is a related condition, which involves perceiving phantom smells that are not actually present, such as detecting the odor of smoke or a strong chemical smell.
Parosmia and phantosmia can be frustrating, but their presence often indicates that the sensory neurons are actively attempting to reconnect and rewire. While the symptoms can last for several months, and sometimes over a year, they are typically temporary and signal a state of active recovery.
Olfactory Training and Other Recovery Methods
For those experiencing lingering smell loss, olfactory training is a non-invasive method designed to stimulate the regenerating nerve cells and encourage correct neural connections. This technique is often described as physical therapy for the nose, leveraging the brain’s neuroplasticity to relearn and correctly identify scents. The standard protocol involves systematically sniffing four distinct categories of odors, typically floral, fruity, spicy, and resinous, twice daily.
The four common scents used are:
- Rose
- Lemon
- Eucalyptus
- Clove
These should be inhaled for about 15 seconds each while actively concentrating on the memory of what the smell should be. After three months, it is recommended to switch to a new set of four odors, such as menthol, thyme, tangerine, and jasmine, to continue stimulating a broader range of olfactory receptors. This consistent, repetitive exposure aims to strengthen the pathways between the nose and the brain.
A physician may also recommend other supportive measures, such as nasal steroid sprays to reduce persistent inflammation in the nasal lining. For persistent anosmia, it is advisable to consult an ear, nose, and throat specialist to rule out other possible causes, such as nasal polyps or chronic sinusitis. The goal of any intervention is to support the natural healing process of the olfactory system.