Overview

Anosmia is the complete inability to perceive smell or odour. It can be temporary or permanent, and either acquired, due to infections, trauma, or neurological disease, or congenital, present from birth.

The sense of smell depends on a complex interaction between the nasal structures and the brain. Inside the nasal cavity are specialised sensory cells called olfactory neurons (or odour receptors), which detect scent molecules in the air. These sensory neurons send signals to the olfactory bulb, located at the brain’s base. From there, the signal is relayed to higher-order brain regions involved in processing odours, such as the piriform cortex, entorhinal cortex, amygdala, and hippocampus, which contribute to odour recognition, memory, and emotional association.

Disruptions at any point along this pathway, from the olfactory epithelium to the central processing centres, can impair the sense of smell. Such disruptions may result from inflammation, obstruction, infection, head trauma, neurodegenerative conditions, or direct damage to neural structures.

While often overlooked, anosmia, a complete loss of smell, has far-reaching implications that extend well beyond the inability to perceive odours. Most individuals only become aware of how critical this sense is once it's impaired. Affecting approximately 5% of the population, anosmia is more prevalent in men, increases with age, and shows higher rates among Black individuals compared to white populations.⁵

Anosmia can result from a variety of causes, including viral infections (notably COVID-19), head trauma, neurodegenerative disorders such as Parkinson’s and Alzheimer’s disease, genetic conditions, and tumours affecting the nasal passages or brain. While some cases are temporary and resolve once the underlying issue is treated, others may lead to permanent loss of smell. Among the most common causes are sinonasal diseases like chronic rhinosinusitis, post-traumatic olfactory damage, and idiopathic cases with no identifiable origin. The condition gained widespread recognition during the COVID-19 pandemic, where sudden smell loss became a hallmark symptom.

In many cases, anosmia or hyposmia improves once the underlying cause is identified and treated. However, if left unaddressed, smell loss may become permanent. Early medical evaluation is essential to prevent long-term deficits and to optimise outcomes. Managing anosmia often requires a team-based, interprofessional approach, involving otolaryngologists, neurologists, primary care providers, and rehabilitation specialists.^1,2

Burden and impact of anosmia

Anosmia, or the loss of the ability to perceive odours, has implications that reach well beyond a simple sensory deficit. It can cause profound psychological distress, contributing to feelings of social and physical vulnerability, emotional isolation, and even victimisation. Numerous individuals describe experiencing emotional dullness, diminished interest in eating, and a significant reduction in overall quality of life.

The sense of smell plays a vital role in maintaining overall health, nutritional status, and personal safety. It stimulates appetite and plays a vital role in detecting environmental hazards. In its absence, individuals may face nutritional challenges, including loss of appetite and malnutrition, particularly among older adults. A reduced ability to enjoy food is common, driven not by loss of taste per se, but by the diminished experience of flavour, which heavily relies on smell. This can result in weight fluctuations, with some gaining weight (particularly younger individuals, who may overeat to seek pleasure) and others losing weight due to reduced food enjoyment. Additionally, the inability to detect spoiled or rancid food, gas leaks, smoke, or toxic fumes significantly compromises personal safety and increases the risk of accidental harm.

The Warwick Olfaction Research Group initiated a pioneering study in the early 1980s to examine the far-reaching effects of anosmia, developing a questionnaire directed at those living with the condition. The responses highlighted recurring themes of unhappiness, emotional blunting, and frustration, not only with the condition itself but also with how it was dismissed or misunderstood by both society and the medical community. Many participants described a sense of being invisible and unsupported, even in clinical settings, despite the condition’s often sudden onset and poor prognosis for recovery.

These findings emphasise that anosmia is not merely a minor inconvenience but a condition with substantial psychosocial and medical consequences. Greater awareness, clinical attention, and empathetic engagement are essential to improving outcomes and quality of life for those affected^3,4

Treatment and management of anosmia

The treatment of anosmia depends primarily on identifying and addressing the underlying cause, as anosmia is a symptom rather than a standalone diagnosis. A thorough clinical evaluation is essential to determine the root condition contributing to the loss of smell.

In cases where inflammatory or obstructive conditions are responsible, such as allergic rhinitis, sinusitis, nasal polyps, or anatomical deformities, management focuses on reducing inflammation and relieving obstruction. Intranasal glucocorticoids, antihistamines, and systemic steroids are commonly prescribed, while antibiotics such as ampicillin may be used in bacterial infections. If conservative medical treatment is ineffective, surgical intervention may be required, especially for chronic sinus disease or persistent polyps.

When anosmia is associated with neurodegenerative disorders like Alzheimer’s or Parkinson’s disease, management typically targets the underlying neurological condition, although complete restoration of olfactory function is often not achievable.

In some cases, particularly when the cause remains unclear or results from head trauma or neural injury, specific treatment options may be limited. Although there is no targeted therapy for olfactory nerve damage, the olfactory system possesses some regenerative potential. Neurons may recover function over time, but regeneration can span from days to several years and varies between individuals. Complete recovery is not guaranteed.^1,4

Olfactory training (OT) as a therapeutic approach  

An important non-pharmacological approach for anosmia is olfactory training (OT), which involves structured exposure to specific odours over several weeks. Traditionally, OT includes four distinct scents—clove, citronella, eucalyptus, and phenylethyl alcohol. Patients are instructed to gently sniff each scent for approximately 10 seconds, twice daily, over 12 weeks. This regimen forms the standard protocol; however, variations such as prolonging training duration or rotating odorants have been associated with improved outcomes.⁸

Research shows that OT may stimulate olfactory nerve regeneration, promote functional brain reorganisation, and increase olfactory bulb volume, thereby supporting recovery in individuals with olfactory dysfunction.⁸ It has demonstrated significant benefits in patients with COVID-19-related anosmia, in both acute and chronic stages.⁶ Moreover, studies have found OT to be beneficial even for healthy individuals, enhancing their olfactory sensitivity through consistent scent exposure.

While the exact mechanisms of OT are still under investigation, several theories suggest that it may facilitate neural regeneration, encourage plasticity in olfactory brain regions, and enhance olfactory bulb structure and function. These neuroplastic changes are believed to underpin its therapeutic effects.

Multiple studies have confirmed OT’s effectiveness, especially in post-infectious olfactory loss. For example, individuals recovering from infection have shown improvements in odour detection and identification following OT. It has also been helpful in cases of anosmia due to upper respiratory infections and head trauma.

Further improvements have been observed when patients use modified odorants or prolong the training period, indicating a dose-response relationship and the adaptability of the olfactory system.

Clinical guidelines also support OT. The British Rhinological Society recommends OT for any patient with persistent olfactory dysfunction lasting over two weeks post-COVID-19. A meta-analysis of multiple studies up to May 2022 confirmed that OT significantly improved olfactory scores in patients with post-COVID-19 smell loss. The analysis showed that OT was effective in both acute and chronic stages of dysfunction, though patients who began training during the acute phase experienced slightly greater benefits. Interestingly, the duration of OT, whether shorter or longer than two months, did not significantly affect outcomes, suggesting flexibility in treatment timelines.⁸

Supporting this, a small case series involving eight adults with persistent post-COVID-19 anosmia showed measurable improvements after a 30-day OT protocol using plant-based essential oils. Notable gains were seen in self-reported olfactory ability and odour recognition, though improvements in overall quality of life were more modest. These early findings indicate that even short-term OT may aid individuals with prolonged post-viral smell dysfunction.⁷

Long-term studies have also highlighted the value of sustained training. Patients who performed OT consistently over extended periods after COVID-19 infection showed greater recovery and higher test scores than those who did not. 

In summary, OT is a safe, accessible, and evidence-based intervention for both acute and chronic smell loss. While early intervention yields optimal results, even delayed OT can produce meaningful improvements. By promoting neural recovery and enhancing olfactory perception, OT plays a vital role in improving the quality of life and reducing the burden of anosmia.

Summary

Anosmia, the complete loss of smell, may arise from various causes, including infections, trauma, and neurological conditions. Among the emerging therapeutic strategies, olfactory training (OT) stands out as a non-pharmacological, evidence-based approach to enhance olfactory recovery.  Studies have demonstrated OT’s effectiveness in post-infectious, post-traumatic, and idiopathic anosmia, with additional evidence suggesting benefits even in healthy individuals. Its use in COVID-19-related smell loss is particularly well-supported, with both short- and long-term interventions showing measurable improvement. OT is safe, accessible, and adaptable, making it a valuable therapeutic option for improving olfactory function and overall quality of life in individuals affected by anosmia.