Upper respiratory infections (URIs) are infections affecting the nose, sinuses, and throat, commonly including the common cold and flu. They frequently cause a temporary loss of smell (anosmia) and taste (ageusia). A diminished sense of taste is usually a consequence of the loss of smell, as approximately 80% of what is perceived as taste is derived from the sense of smell. Many individuals with a URI experience some degree of olfactory dysfunction.
How Upper Respiratory Infections Affect Sensation
The mechanism by which a URI diminishes the sense of smell involves two pathways. The first is conductive loss, where inflammation and physical blockage prevent odor molecules from reaching the sensory tissue. During an acute infection, nasal passages swell and excessive mucus production occurs, creating a physical barrier. This prevents odorants from reaching the olfactory epithelium, the tissue high in the nasal cavity that contains the smell receptors. This form of smell loss is temporary and resolves quickly once congestion clears.
The second pathway involves sensorineural loss, which is a direct injury to the delicate olfactory system. Certain viruses can cause inflammation or damage to the supporting cells that maintain the health of the olfactory sensory neurons. These neurons are responsible for detecting odors and transmitting signals to the brain. When these supporting cells are compromised, the sensory neurons can become dysfunctional, leading to a loss of smell.
While many common cold viruses primarily cause conductive loss, certain pathogens, such as the one responsible for COVID-19, cause significant sensorineural damage, even without severe nasal congestion. This distinction explains why some viral infections result in a quick recovery, while others lead to a more prolonged period of smell dysfunction. The damage to the specialized nerve tissue requires time for the body to repair and regenerate the affected cells.
The Typical Recovery Timeline
For most people experiencing chemosensory loss due to a cold or flu, recovery is fast. The sense of smell usually returns shortly after nasal congestion and inflammation subside. This acute loss resolves within a few days to one or two weeks as the body clears the infection.
When the loss is due to severe post-viral damage to the olfactory neurons, the recovery timeline is extended. Studies show that many individuals with this type of loss recover their sense of smell within one to three months following the initial infection. The ability of the olfactory neurons to regenerate, though slow, drives this eventual improvement.
Smell Training
If the sense of smell does not return quickly, the recovery process can continue for six to twelve months or longer. During this time, patients may experience qualitative changes in smell, such as parosmia, where familiar scents are perceived as distorted or unpleasant. To encourage recovery, smell training (olfactory retraining) is often recommended.
This involves repeated sniffing of strong, distinct odors, such as essential oils of rose, eucalyptus, lemon, and clove. This process stimulates the repair and regeneration of damaged olfactory neurons and pathways. Consistency is important, as improvements may take three months or more to become noticeable.
When Loss of Taste or Smell Requires a Doctor Visit
While temporary chemosensory loss after a URI is common, certain circumstances warrant a medical consultation. If the loss of smell or taste does not begin to improve within two weeks after other cold symptoms have disappeared, seek evaluation. If the dysfunction persists beyond four to six weeks, an appointment with a healthcare provider or an ear, nose, and throat specialist (otolaryngologist) is necessary.
A medical professional can rule out other potential causes for the sensory loss that are not related to a typical viral infection. These alternative causes include chronic nasal issues, such as nasal polyps, which physically obstruct the nasal passages. Sudden or isolated loss of smell without any preceding congestion or cold symptoms also requires prompt medical attention.
A doctor visit is also necessary if the loss of smell is accompanied by other unusual symptoms. These red flags include severe, persistent headaches, facial pain or pressure that does not ease, or vision changes. These additional symptoms may indicate a severe sinus infection or, rarely, a neurological problem or a mass that requires immediate diagnosis and treatment.