The loss of the sense of smell, known as anosmia, became one of the most recognizable symptoms of COVID-19 infection worldwide. This sudden inability to detect odors, or a reduced sense of smell (hyposmia), significantly impacts quality of life. The olfactory sense is tied to safety, such as detecting smoke or spoiled food, and emotional well-being. For a substantial portion of those infected, this symptom can persist long after other signs of the illness have resolved, prompting the question of when this sense will return.
The Biological Mechanism of Smell Loss
The mechanism behind COVID-19-related smell loss explains why it is often temporary compared to other types of nerve damage. The SARS-CoV-2 virus does not primarily infect the olfactory sensory neurons, which are the nerve cells that directly detect odor molecules. Instead, the virus targets the supporting cells within the olfactory epithelium, known as sustentacular cells, because they express the ACE2 receptor protein that the virus uses for entry.
Infecting and damaging these sustentacular cells disrupts the environment necessary for the sensory neurons to function correctly. These supporting cells are responsible for providing metabolic and structural maintenance to the olfactory neurons. The resulting inflammation and damage can lead to the retraction of the cilia on the sensory neurons, temporarily disabling their ability to transmit signals to the brain. Since the sensory neurons are spared from direct infection, they can recover their function once the supporting cellular environment is restored, accounting for the rapid recovery seen in many patients.
Typical Recovery Timelines and Statistical Data
For the majority of individuals who experience smell loss during a COVID-19 infection, the sense of smell returns relatively quickly. Olfactory function often begins to improve within two to three weeks of the initial infection. Data suggests that approximately 60 to 70 percent of patients regain their sense of smell, either partially or entirely, within the first four weeks.
The recovery trajectory continues steadily over subsequent months. Around 78 percent of patients experience full recovery within two months, and self-reported data indicates that up to 95 percent of patients regain their function within six months. In cases where the smell loss persists longer, recovery can still occur, demonstrating the remarkable capacity for regeneration in the olfactory system. Studies tracking patients for longer periods show that roughly 90 percent of those who experienced smell loss eventually recover within two years.
The speed of recovery is influenced by the severity of the initial olfactory loss. Individuals who experienced a complete inability to smell (anosmia) tend to have a longer recovery period than those with only a reduced sense of smell (hyposmia). Additionally, persistent olfactory dysfunction is more commonly reported in older patients and females. If the sense of smell has not returned or improved after four weeks, consulting a specialist is recommended to explore therapeutic options and rule out other causes of olfactory dysfunction.
Strategies for Accelerating Olfactory Recovery
The primary strategy for encouraging the return of smell is Olfactory Training, often described as physical therapy for the nose. This process involves regular and repeated exposure to specific odors to stimulate damaged olfactory nerves and promote regeneration. The goal is to leverage the brain’s neuroplasticity to help the nerves regrow and correctly re-establish connections.
The standard protocol involves using four distinct scents, representing the major odor categories. These scents are typically presented as essential oils, which are sniffed in a focused manner twice per day, every day. It is recommended to perform short, deliberate sniffs of each scent for about 10 to 20 seconds, concentrating on recalling the odor in your memory while smelling it.
- Floral (such as rose)
- Fruity (like lemon or orange)
- Spicy (such as clove)
- Resinous (like eucalyptus)
Training should be maintained for a minimum of 12 weeks, and many specialists recommend continuing for several months to maximize the potential for improvement. For those undertaking extended training, a modified approach suggests switching to a new set of four scents after the initial 12 weeks to provide broader sensory stimulation. Other supportive measures, such as nasal corticosteroids, may sometimes be considered, but these are typically reserved for specific cases and require consultation with a medical professional.
Understanding Parosmia and Distorted Smells
As the olfactory nerves begin to regenerate, some people experience a condition called parosmia, which is a distortion of the sense of smell. Instead of being unable to detect an odor, the person perceives familiar smells as highly unpleasant, often described as smelling like garbage, burning, or rotten meat. This qualitative change in smell is distinct from the quantitative loss of anosmia and can be extremely distressing, sometimes leading to significant weight loss due to an aversion to food.
Parosmia is often seen as a sign of recovery, occurring when the regrowing olfactory nerve fibers have not yet correctly connected to the brain’s olfactory bulb. The misfiring of these misaligned connections results in the distorted perception of the odor molecule. This phenomenon commonly begins around three to four months after the initial COVID-19 infection, coinciding with the time frame of nerve regeneration.
Specific items are frequently reported as triggers for parosmia, including coffee, meat, onions, and garlic. Although highly disruptive, the condition generally has a positive prognosis, with symptoms tending to improve gradually over many months as the neural connections mature. Continuing with olfactory training is considered beneficial for individuals with parosmia, as it can help to retrain the brain to correctly interpret the signals sent by the regenerating nerves.