Loss of the sense of smell is usually caused by obstructive olfactory disorders, lesions of the olfactory mucosa and the olfactory nervous system. It may be related to nasal diseases, head trauma, tumors, viral infections, smoking, and certain drugs. 1. Nasal diseases: structural deformities of the nose; obstructive inflammation of the nasal cavity, such as acute and chronic rhinitis, sinusitis, allergic rhinitis, etc.; occupational lesions of the nasal cavity and nasopharynx. The air can not reach the olfactory region through breathing, thus causing olfactory impairment, and in severe cases, loss of sense of smell. 2. Trauma: Trauma to the forehead affects the sense of smell most commonly, usually due to direct injury to the mucous membrane of the olfactory region; fracture of the frontal bone, causing the olfactory nerve through the sieve plate of the sieve bone to be severed; oedema and blood clots caused by trauma can also compress the olfactory bulb and the olfactory nerve channel. 3. Tumors: Tumors in and around the nasal cavity can directly compress the olfactory nerve or olfactory channels, causing an increase in the olfactory threshold. 4. Viral infection: Viral infection can directly damage the olfactory receptors, causing hyposmia or even loss of smell. 5. Smoking: Long-term heavy smoking can damage the olfactory nerve, resulting in loss of smell. 6. Drugs: drugs such as Tabazole, Streptomycin, Lincomycin, etc. can cause olfactory dysfunction, leading to loss of smell. There are many reasons for losing the sense of smell, it is recommended to go to the hospital in time, identify the cause and timely treatment.