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Parosmia

What Is It, Causes, Treatment, and More

Author:Georgina Tiarks

Editors:Alyssa Haag,Emily Miao, PharmD

Illustrator:Jessica Reynolds, MS

Copyeditor:David G. Walker


What is parosmia?

Parosmia refers to an olfactory dysfunction characterized by a distorted sense of smell in which smells that were previously enjoyable may now be deemed foul. It is often experienced following a viral infection. Olfactory dysfunctions are categorized into quantitative (e.g., anosmia and hyposmia) disorders, which measure an objective variation in the ability to smell, and qualitative (e.g., parosmia and phantosmia) disorders, which describe a subjective change to smell quality.  Anosmia describes the complete loss of the sense of smell, and hyposmia occurs when there is a decreased sense of smell. Comparatively, phantosmia occurs when an individual may be able to detect an odor that is not present. 

 In general, smell is sensed through the olfactory bulb, which resides in the nasal cavity on the anterior surface of the frontal lobe (i.e., lobe of the brain that controls behavior and emotion). Neurons of the olfactory bulb send signals to the primary olfactory cortex within the cerebral hemispheres (i.e., the outermost portion of the brain). Damage to the olfactory bulb or disruption along this neuronal pathway may cause parosmia.

Close up of inflamed olfactory bulb and profile of a man smelling a rose and thinking it smells like garbage.

What causes parosmia?

Parosmia is often caused by damage to the olfactory bulbs. Most commonly, this occurs after a nasal or respiratory infection, causing post-infectious olfactory dysfunction (PIOD). Parosmia is a common symptom of COVID-19, especially as a long-term effect. Parosmia may also arise due to head trauma, environmental exposures, persistent dry mouth, normal aging, brain tumors, or central nervous system disorders (e.g., Parkinson’s disease, Alzheimer disease, multiple sclerosis, or malignancy). Occasionally, there may be no definitive cause of parosmia

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How does COVID-19 cause parosmia?

Parosmia may occur after infection with the novel coronavirus (i.e., COVID-19). Although the mechanism is not completely clear, parosmia after COVID-19 is thought to be due to damage to the neuroepithelium (i.e., the stem cells that become neurons) and olfactory bulb (i.e., the nerves responsible for transmitting smell) through both viral injury and continuous inflammation. Studies hypothesize that this neuronal death causes an alteration of receptor-mediated targets, thereby altering the ability to properly classify odors.

How is parosmia treated?

There are a variety of different treatment options for parosmia. Zinc and vitamin supplements have been known to be effective treatment methods if there is a suspected or confirmed malnutrition. Ongoing research into smell training (i.e., retraining one’s sense of smell) has also shown to be promising. Individuals exposed to four different odors twice daily for several months may lead to gradual improvement in their sense of smell. In some circumstances, smell may be regained spontaneously due to neuronal regeneration. Though not widely used, in some cases of parosmia and phantosmia, medical professionals may also prescribe neuroleptic or antiepileptic medications, such as valproate or gabapentin, to possibly promote neuronal regeneration.

How long does parosmia last?

Individuals who experience parosmia usually have symptoms longer than two weeks. Research shows that, on average, an episode of parosmia lasts for about three months, with some cases persisting up to six months. However, the length of symptoms depends largely on the underlying etiology. 

What are the most important facts to know about parosmia?

Parosmia is a distortion of the normal sense of smell, where an odor that is usually pleasant may now smell bad. Parosmia typically occurs due to damage to the neuroepithelium and olfactory bulbs. This often transpires after viral infections but may also be a result of brain tumors, head trauma, or neurodegenerative disorders. Treatment of parosmia may include vitamins, minerals, olfactory training, supportive care, and addressing the underlying cause. However, in some cases, it can spontaneously resolve. On average, parosmia lasts for around three months.

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Related links

Anatomy of the olfactory (CN 1) and optic (CN II) nerves
Olfactory transduction and pathways
Cranial nerves
COVID-19 (Coronavirus Disease 19)
Taste and tongue

Resources for research and reference

Di Stadio, A., D’Ascanio, L., La Mantia, I., Ralli, M., & Brenner, M. J. (2022). Parosmia after COVID-19: Olfactory training, neuroinflammation and distortions of smell. European Review for Medical and Pharmacological Sciences, 26(1), 1–3. DOI: 10.26355/eurrev_202201_27739

Liu, D. T., Sabha, M., Damm, M., Philpott, C., Oleszkiewicz, A., Hähner, A., & Hummel, T. (2020). Parosmia is associated with relevant olfactory recovery after olfactory training. The Laryngoscope, 131(3), 618–623. DOI: 10.1002/lary.29277

Martin, J. H. (2021). Chemical Senses: Taste and Smell. In Neuroanatomy: Text and Atlas (5th ed.). McGraw Hill. 

Ogawa, T., Takezawa, K., Shimizu, S., & Shimizu, T. (2014). Valproic acid promotes neural regeneration of olfactory epithelium in adult mice after methimazole-induced damage. American Journal of Rhinology & Allergy, 28(2), e95–e99. DOI: 10.2500/ajra.2014.28.4027

Raad, N., Ghorbani, J., Safavi Naeini, A., Tajik, N., & Karimi-Galougahi, M. (2021). Parosmia in patients with COVID-19 and olfactory dysfunction. International Forum of Allergy & Rhinology, 11(10), 1497–1500. DOI: 10.1002/alr.22818

Rashid, R. A., Alaqeedy, A. A., & Al-Ani, R. M. (2021). Parosmia due to COVID-19 disease: A 268 case series. Indian Journal of Otolaryngology and Head & Neck Surgery, 1–8. DOI: 10.1007/s12070-021-02630-9

Ropper, A. H., Samuels, M. A., Klein, J. P., & Prasad, S. (2019). Disorders of Smell and Taste. In Adams and Victor’s Principles of Neurology (11th ed.). McGraw-Hill Education. Retrieved from accessmedicine.mhmedical.com/content.aspx?aid=1162590713