Anosmia, or loss of the sense of smell, varies from congenital to acquired, and can be temporary to permanent. Congenital olfactory loss is an extremely rare problem, and so far only two genes have been found whose mutations may be responsible for its occurrence. Acquired anosmia, on the other hand, is much more common.

Sometimes the loss of the sense of smell occurs due to natural changes that occur with age in the human body. Typically the sense of smell in the elderly is weakened, but it can also be completely lost due to, for example, progressive loss of olfactory cells or a reduction in the number of receptors in the olfactory bulb.

Infections, especially upper respiratory tract infections, are a possible cause of anosmia. There is an increased risk of olfactory loss in patients who have polyps in the nasal cavity, but also in those who have a curvature of the nasal septum. Sometimes anosmia occurs in the course of allergic rhinitis. In 2020, it was found that loss of smell - often together with loss of taste - can be a symptom of COVID-19.

However,loss of smell can also occur in association with neurological conditions. In particular, neurodegenerative diseases such as Parkinson's disease, amyotrophic lateral sclerosis or Huntington's disease, among others, can lead to anosmia.

Sometimes, it is not really possible to determine the cause of the loss of the sense of smell, in which case idiopathic anosmia is diagnosed. Sometimes it is the patients themselves who contribute to the olfactory loss - the problem can occur in people who use drugs intranasally, and it can also be caused by smoking.