Effect of Olfactory Training in COVID-19 Related Olfactory Dysfunction-A Placebo-Controlled Trial

OBJECTIVES: Olfactory training is a recommended treatment in patients with olfactory dysfunction after COVID-19. However, the effectiveness of olfactory training in COVID-19-induced olfactory dysfunction has been studied; only a few studies have included a placebo group to compare the effect of traditional olfactory training with odor-free olfactory training. In this randomized placebo-controlled trial, we evaluate the effect of olfactory training with essential oils versus olfactory training with placebo oils in patients with post-COVID-19 olfactory dysfunction.

METHODS: Sixty-five patients with COVID-19 induced olfactory dysfunction were randomized to the intervention or control group. Patients in the intervention group performed classic olfactory training with four essential oils (orange, lavender, clove, and peppermint) twice daily for 12 weeks, while the control group performed the same olfactory training but with odor-free placebo oils. Olfactory function was assessed using the Sniffin Sticks extended TDI test at baseline and at follow-up after the 12-week intervention.

RESULTS: The intervention group did not significantly improve their olfactory function compared to the control group after olfactory training. Additionally, no associations were found between changes in olfactory function after olfactory training and age, gender, rhinitis, time since infection, or pre-infection vaccine. A regression analysis showed that the presence of parosmia was associated with reduced effectiveness of olfactory training.

CONCLUSIONS: This study found that 12 weeks of classical olfactory training is not effective in the treatment of COVID-19 induced olfactory dysfunction. Moreover, the study indicates that parosmia reduces the effect of olfactory training significantly.

TRIAL REGISTRATION: Clinicaltrials.gov registration number: NCT05539560.