Olfactory Training After COVID: What the Research Shows

COVID-19 and smell loss: scope of the problem

Early in the COVID-19 pandemic, clinicians around the world began reporting that a notable proportion of infected individuals experienced sudden changes in their sense of smell and taste, often as one of the first or only symptoms.

A large multicenter European study published in 2020 by Lechien et al. collected data from 417 patients with mild-to-moderate COVID-19 across twelve hospitals in four European countries. The researchers reported that 85.6% of participants reported some degree of olfactory dysfunction. Among those with olfactory symptoms, 79.6% reported anosmia (complete loss) and 20.4% reported hyposmia (partial reduction). The study noted that olfactory dysfunction appeared early in the course of illness and, in some cases, was the presenting symptom.

Citation: Lechien JR, Chiesa-Estomba CM, De Siati DR, et al. Olfactory and gustatory dysfunctions as a clinical presentation of mild-to-moderate forms of the coronavirus disease (COVID-19): a multicenter European study. European Archives of Oto-Rhino-Laryngology. 2020;277(8):2251-2261. doi:10.1007/s00405-020-05965-1

Subsequent studies from research groups worldwide reported similar findings, with prevalence estimates for olfactory dysfunction in COVID-19 varying across studies depending on how smell loss was measured (self-report vs. psychophysical testing), the severity of illness, the viral variant, and the population studied. A systematic review and meta-analysis by Agyeman et al. (2020) that pooled data from multiple early studies estimated that olfactory dysfunction affected approximately 41% of COVID-19 patients based on available data at that time, though the authors noted that studies using objective testing instruments tended to report higher rates than those relying solely on self-report.

Citation: Agyeman AA, Chin KL, Landersdorfer CB, Liew D, Ofori-Asenso R. Smell and taste dysfunction in patients with COVID-19: a systematic review and meta-analysis. Mayo Clinic Proceedings. 2020;95(8):1621-1631. doi:10.1016/j.mayocp.2020.05.030

Recovery patterns: what the published data shows

For many individuals with COVID-19-related smell loss, olfactory function returned within weeks. However, a clinically significant subset experienced prolonged or persistent dysfunction.

Early observational data from Hopkins et al. (2020) followed individuals in the United Kingdom who developed new-onset anosmia during the pandemic. The researchers reported that many participants experienced some recovery within the first few weeks, but that a meaningful proportion still reported persistent olfactory symptoms at the time of follow-up.

Citation: Hopkins C, Surda P, Kumar N. Presentation of new onset anosmia during the COVID-19 pandemic. Rhinology. 2020;58(3):295-298. doi:10.4193/Rhin20.116

Longer-term follow-up studies provided further data on recovery trajectories. Boscolo-Rizzo et al. conducted a prospective study tracking olfactory and gustatory function in one of the first European cohorts of COVID-19 patients over a 12-month period. Their findings, published in 2022, reported that while the majority of participants showed improvement over the follow-up period, a subset continued to report persistent olfactory dysfunction at one year. The researchers also noted that parosmia (distorted smell perception) emerged as a distinct concern for some participants during the recovery period, sometimes appearing weeks or months after the initial infection.

Citation: Boscolo-Rizzo P, Hummel T, Hopkins C, et al. High prevalence of long-term olfactory, gustatory, and chemesthesis dysfunction in post-COVID-19 patients: a matched case-control study with one-year follow-up using a comprehensive psychophysical evaluation. Rhinology. 2022;60(6):517-527. doi:10.4193/Rhin22.230

These and other studies contributed to an understanding that post-COVID olfactory dysfunction follows a general pattern: a majority of individuals experience recovery within 1–3 months, but an estimated 5–20% (depending on the study and methodology) continue to experience persistent olfactory impairment at 6 months or beyond. The wide range in these estimates reflects differences in study populations, measurement methods, viral variants, and follow-up periods.

Why post-COVID smell loss is relevant to olfactory training research

The connection between COVID-19 and olfactory training is straightforward: COVID-19 is a viral infection, and smell loss that persists after recovery from a viral infection is classified as post-infectious olfactory dysfunction. The same category that has the largest body of published research on olfactory training.

The foundational olfactory training studies that predated the pandemic were conducted primarily in individuals with post-infectious olfactory loss (caused by colds, influenza, and other upper respiratory infections) and post-traumatic olfactory loss (caused by head injury). The Hummel et al. (2009) study, the Konstantinidis et al. (2013) study, the Damm et al. (2014) multicenter study, and the Konstantinidis et al. (2016) long-term study all included or specifically focused on post-infectious populations.

This existing evidence base meant that when COVID-19 emerged as a significant new cause of post-infectious olfactory loss, there was already a body of published literature examining olfactory training in the same general category of smell disorder. Researchers and clinicians did not need to start from zero. They could reference a decade of prior work while also conducting new studies specific to post-COVID populations.

Clinical guidance during and after the pandemic

As evidence of persistent post-COVID olfactory dysfunction accumulated, medical professional organizations and expert groups published guidance on management approaches.

In the United Kingdom, the British Rhinological Society and ENT UK were among the first organizations to issue recommendations, identifying olfactory training as a low-risk, evidence-based approach that individuals with post-COVID smell loss could initiate at home. Their guidance referenced the existing olfactory training literature and recommended it as a first-line strategy for patients with persistent post-infectious olfactory dysfunction following COVID-19.

A clinical consensus statement on the treatment of postinfectious olfactory dysfunction, published in 2021 by Addison et al. in The Journal of Allergy and Clinical Immunology, brought together an international panel of clinicians and researchers. The panel reviewed available treatment evidence across multiple modalities and identified olfactory training as having the most consistent evidence base for post-infectious olfactory loss. The consensus statement recommended olfactory training as a first-line approach, noting its safety profile, the absence of known significant adverse effects, and the body of supporting literature.

Citation: Addison AB, Wong B, Ahmed T, et al. Clinical Olfactory Working Group consensus statement on the treatment of postinfectious olfactory dysfunction. The Journal of Allergy and Clinical Immunology. 2021;147(5):1704-1719. doi:10.1016/j.jaci.2020.12.641

The positioning of olfactory training as a first-line approach reflects several factors discussed in the clinical literature: the existing evidence base from pre-COVID studies, the non-invasive nature of the practice, the ability for individuals to perform it independently at home, the absence of significant reported adverse effects in the published literature, and the limited alternatives with strong evidence for post-infectious olfactory loss specifically.

It is important to note that "first-line" in this context means it is the approach that clinical experts recommend trying first. It does not mean it is guaranteed to work for every individual, nor does it mean it is the only approach that has been studied.

Studies of olfactory training in post-COVID populations

While the pre-existing evidence base was drawn from earlier causes of post-infectious olfactory loss, several research groups have since conducted studies examining olfactory training specifically in individuals with post-COVID olfactory dysfunction.

D'Ascanio et al. (2021). Randomized Controlled Trial

Researchers in Italy conducted a randomized controlled trial enrolling patients with persistent olfactory dysfunction following confirmed COVID-19 infection. Participants were randomized to either an olfactory training group following the standard four-odor protocol or a control group. Olfactory function was assessed using psychophysical testing before and after the training period.

The researchers reported that participants in the olfactory training group showed statistically significant improvements in olfactory test scores compared to the control group. This was one of the first published RCTs to examine the standard olfactory training protocol specifically in a post-COVID population.

Citation: D'Ascanio L, Vitelli F, Cingolani C, et al. Randomized clinical trial "olfactory training vs. budesonide nasal spray in post-COVID-19 patients with persistent olfactory dysfunction." European Archives of Oto-Rhino-Laryngology. 2022;279(12):5761-5767. doi:10.1007/s00405-022-07449-y

Altundag et al. (2022). Modified Olfactory Training Protocol

Building on their earlier work with modified olfactory training protocols (which introduced the concept of rotating scent sets after an initial training period), Altundag and colleagues conducted a study specifically in post-COVID patients. This research examined whether a modified protocol (in which the four training scents were changed every 12 weeks to introduce novel stimuli) offered any differences compared to the standard fixed-scent protocol in a post-COVID population.

The researchers reported that participants in the modified training group showed improvements on olfactory testing. The concept of modified or rotational olfactory training has been discussed in the literature as a potential way to provide continued novelty to the olfactory system over longer training durations.

Citation: Altundag A, Yilmaz E, Kesimli MC. Modified olfactory training is an effective treatment method for COVID-19 induced parosmia. The Laryngoscope. 2022;132(7):1433-1438. doi:10.1002/lary.30036

Systematic Reviews of Post-COVID Olfactory Training

As individual studies accumulated, systematic reviews began aggregating the post-COVID olfactory training evidence. Iannuzzi et al. (2022) published a systematic review in Pathogens that examined published studies of olfactory training specifically in COVID-19 patients. The review identified multiple studies that had investigated the practice in post-COVID populations and concluded that the available evidence was consistent with the findings from the pre-COVID olfactory training literature.

Citation: Iannuzzi L, Salzo AE, Angarano G, Palmieri VO, Portincasa P, Saracino A, Gelardi M, Dibattista M, de Candia M. Gaining and losing smell during the COVID-19 pandemic: olfactory training as a possible treatment. Pathogens. 2022;11(4):437. doi:10.3390/pathogens11040437

These COVID-specific studies are important because, while COVID-19 shares the general category of "post-infectious" olfactory loss with other viral infections, the specific mechanisms of how SARS-CoV-2 affects the olfactory system may differ from earlier viruses. Having data from post-COVID populations specifically (in addition to the broader post-infectious evidence base) provides a more complete picture.

How COVID-19 affects the olfactory system

Research published since the start of the pandemic has examined the mechanisms by which SARS-CoV-2 may cause olfactory dysfunction. While a full review of the virology is beyond the scope of this article, several key findings from the published literature are relevant to understanding the potential role of olfactory training.

Early in the pandemic, some researchers hypothesized that the virus might directly damage olfactory sensory neurons. However, subsequent research, including work by Brann et al. (2020) published in Science Advances, found that the ACE2 receptor (the primary entry point for the SARS-CoV-2 virus) is not expressed on olfactory sensory neurons themselves, but is highly expressed on sustentacular (supporting) cells in the olfactory epithelium.

Citation: Brann DH, Tsukahara T, Weinreb C, et al. Non-neuronal expression of SARS-CoV-2 entry genes in the olfactory system suggests mechanisms underlying COVID-19-associated anosmia. Science Advances. 2020;6(31):eabc5801. doi:10.1126/sciadv.abc5801

This finding suggested that the virus may disrupt olfactory function primarily through damage to the supporting cells and surrounding tissue rather than by directly destroying the olfactory neurons, which is relevant because it implies that the neural pathways may remain partially intact or capable of recovery in many cases. The olfactory epithelium has a well-documented capacity for regeneration, and supporting the recovery process through structured odor exposure is the conceptual basis for olfactory training.

Researchers have also noted that the mechanisms of COVID-19-related smell loss may involve inflammation of the olfactory epithelium, disruption of the mucus layer that odor molecules must penetrate to reach receptor cells, and potential secondary effects on central olfactory processing in the brain. These mechanisms are still being studied, and the relative contribution of each may vary across individuals.

Parosmia: a distinct post-COVID concern

Parosmia (the perception of distorted smells, where familiar odors smell different than expected, often in an unpleasant way) became a widely reported phenomenon during the pandemic. While parosmia was documented in the medical literature before COVID-19, the sheer number of individuals experiencing post-COVID olfactory dysfunction brought increased attention to this particular symptom.

Published reports described parosmia as sometimes emerging during the recovery phase, weeks or months after the initial infection, rather than at the time of acute illness. Some researchers have interpreted this pattern as a possible sign that olfactory neurons are regenerating but not yet forming correct connections. The distorted perception potentially reflecting new neurons that have not yet been properly integrated into the olfactory circuitry.

Several of the post-COVID olfactory training studies have included participants with parosmia. The Altundag et al. (2022) study referenced above specifically examined modified olfactory training in post-COVID patients experiencing parosmia. While the evidence base for olfactory training's effect on parosmia specifically is smaller than for anosmia or hyposmia, it is a topic of active investigation in the research community.

Individuals experiencing parosmia should be aware that this is a recognized condition documented in the published medical literature, not an unusual or concerning finding in the context of post-infectious olfactory recovery. Consulting with an ENT specialist or otolaryngologist can provide further guidance.

What about other treatments studied for post-COVID smell loss?

Olfactory training is not the only approach that has been studied in the context of post-COVID olfactory dysfunction. The medical literature has examined several other interventions, and understanding the broader treatment landscape is useful for context.

Intranasal corticosteroids. Some studies have examined whether nasal steroid sprays (such as budesonide or mometasone) accelerate olfactory recovery after COVID-19. Results in the published literature have been mixed. The D'Ascanio et al. (2022) study referenced above compared olfactory training to budesonide and reported findings favoring olfactory training. Some clinical guidelines discuss short courses of intranasal corticosteroids as a possible adjunctive approach, but the evidence is not consistent. Systemic (oral) corticosteroids have been studied as well, with the literature generally noting concerns about their side effect profile relative to the limited evidence of benefit for olfactory recovery specifically.

Omega-3 fatty acids. Some studies have explored whether omega-3 supplementation may support olfactory recovery, based on the anti-inflammatory properties of omega-3 fatty acids and their role in neural tissue repair. Published results have been preliminary and findings have varied.

Platelet-rich plasma (PRP). A small number of studies have examined the injection of PRP into the olfactory cleft as a potential treatment. This is an experimental approach with limited published data.

Theophylline. Intranasal theophylline has been investigated in some published studies for post-viral olfactory loss. The evidence base remains limited.

The Addison et al. (2021) clinical consensus statement reviewed the available evidence for multiple treatment modalities and identified olfactory training as having the most consistent evidence base for post-infectious olfactory loss. The authors noted that for most other interventions, the published evidence was either limited, mixed, or of lower quality. This assessment reflects the state of the published literature and is not a statement about the efficacy of any individual treatment for any individual patient.

Practical considerations for olfactory training after COVID

For individuals with persistent olfactory dysfunction following COVID-19, the published literature and clinical guidance documents suggest the following general framework. These are summaries of recommendations discussed in the medical literature, not medical advice from The Olfactory Training Kit or Advanced Rx.

Consult a healthcare provider. Individuals with persistent smell loss should discuss their symptoms with their physician or an ENT specialist. Persistent olfactory dysfunction can have multiple causes, and a clinician can help determine whether further evaluation is appropriate.

Begin training with the standard protocol. The published studies consistently use the four-odor, twice-daily protocol established by Hummel et al. (2009). Training for a minimum of 12 weeks is consistent with the original study design and is the minimum duration recommended in most clinical guidance.

Maintain consistency. Adherence to the daily protocol is repeatedly identified in the literature as a relevant factor. Researchers emphasize that olfactory training is a sustained practice, not a brief intervention.

Consider longer training durations. The Konstantinidis et al. (2016) long-term study reported that 56 weeks of training was associated with greater improvement than 16 weeks. Several clinical guidance documents note that longer training durations may be beneficial, particularly for individuals who have not observed changes in the initial period.

Use high-quality, concentrated odorants. The Damm et al. (2014) multicenter study specifically demonstrated that odor concentration is a relevant variable, with high-concentration odors associated with greater improvement than very low-concentration odors. Using GC/MS-verified essential oils provides assurance of chemical purity and concentration.

Be aware that recovery timelines vary. The published literature consistently shows that individual outcomes vary. Some participants in the studies showed measurable changes relatively early, while others showed changes only after longer training periods, and some did not show measurable changes on the instruments used in the studies. Post-COVID olfactory dysfunction specifically appears to follow variable recovery trajectories.

References

1. Lechien JR, Chiesa-Estomba CM, De Siati DR, et al. Olfactory and gustatory dysfunctions as a clinical presentation of mild-to-moderate forms of the coronavirus disease (COVID-19): a multicenter European study. European Archives of Oto-Rhino-Laryngology. 2020;277(8):2251-2261. doi:10.1007/s00405-020-05965-1
2. Agyeman AA, Chin KL, Landersdorfer CB, Liew D, Ofori-Asenso R. Smell and taste dysfunction in patients with COVID-19: a systematic review and meta-analysis. Mayo Clinic Proceedings. 2020;95(8):1621-1631. doi:10.1016/j.mayocp.2020.05.030
3. Hopkins C, Surda P, Kumar N. Presentation of new onset anosmia during the COVID-19 pandemic. Rhinology. 2020;58(3):295-298. doi:10.4193/Rhin20.116
4. Boscolo-Rizzo P, Hummel T, Hopkins C, et al. High prevalence of long-term olfactory, gustatory, and chemesthesis dysfunction in post-COVID-19 patients: a matched case-control study with one-year follow-up using a comprehensive psychophysical evaluation. Rhinology. 2022;60(6):517-527. doi:10.4193/Rhin22.230
5. Brann DH, Tsukahara T, Weinreb C, et al. Non-neuronal expression of SARS-CoV-2 entry genes in the olfactory system suggests mechanisms underlying COVID-19-associated anosmia. Science Advances. 2020;6(31):eabc5801. doi:10.1126/sciadv.abc5801
6. Addison AB, Wong B, Ahmed T, et al. Clinical Olfactory Working Group consensus statement on the treatment of postinfectious olfactory dysfunction. The Journal of Allergy and Clinical Immunology. 2021;147(5):1704-1719. doi:10.1016/j.jaci.2020.12.641
7. D'Ascanio L, Vitelli F, Cingolani C, et al. Randomized clinical trial "olfactory training vs. budesonide nasal spray in post-COVID-19 patients with persistent olfactory dysfunction." European Archives of Oto-Rhino-Laryngology. 2022;279(12):5761-5767. doi:10.1007/s00405-022-07449-y
8. Altundag A, Yilmaz E, Kesimli MC. Modified olfactory training is an effective treatment method for COVID-19 induced parosmia. The Laryngoscope. 2022;132(7):1433-1438. doi:10.1002/lary.30036
9. Iannuzzi L, Salzo AE, Angarano G, et al. Gaining and losing smell during the COVID-19 pandemic: olfactory training as a possible treatment. Pathogens. 2022;11(4):437. doi:10.3390/pathogens11040437
10. Hummel T, Rissom K, Reden J, Hähner A, Weidenbecher M, Hüttenbrink KB. Effects of olfactory training in patients with olfactory loss. The Laryngoscope. 2009;119(3):496-499. doi:10.1002/lary.20101
11. Damm M, Pikart LK, Reimann H, et al. Olfactory training is helpful in postinfectious olfactory loss: a randomized, controlled, multicenter study. The Laryngoscope. 2014;124(4):826-831. doi:10.1002/lary.24340
12. Konstantinidis I, Tsakiropoulou E, Constantinidis J. Long term effects of olfactory training in patients with post-infectious olfactory loss. Rhinology. 2016;54(2):170-175. doi:10.4193/Rhino15.264

This article is provided for informational and educational purposes only. It is not medical advice and is not intended to diagnose, treat, cure, or prevent any disease. The studies referenced above represent published scientific research; their inclusion does not constitute a claim that any product will produce the same results. COVID-19 is a serious illness, individuals who suspect they have COVID-19 or are experiencing persistent symptoms should consult with a qualified healthcare provider.

The Olfactory Training Kit is assembled at Advanced Rx, a licensed pharmacy in Fort Washington, Pennsylvania.