Common causes
Nasal and sinus disease (e.g., allergic or vasomotor rhinitis, chronic sinusitis, nasal polyps, adenoid hypertrophy)
Upper respiratory infection
Head trauma (e.g., frontal skull fracture, occipital injury, nasal fracture)
Cigarette smoking
Neurodegenerative disease (e.g., Alzheimer's disease, Parkinson's disease, multiple sclerosis)
Age

Less common causes
Medications (see Table 2)
Cocaine abuse (intranasal)
Toxic chemical exposure (e.g., benzene, benzol, butyl acetate, carbon disulfide, chlorine, ethyl acetate, formaldehyde, hydrogen selenide, paint solvents, sulfuric acid, thrichloroethylene)
Industrial agent exposure (e.g., ashes, cadmium, chalk, chromium, iron carboxyl, lead, nickel, silicone dioxide)
Nutritional factors (e.g., vitamin deficiency [A, B₆, B₁₂], trace metal deficiency [zinc, copper], malnutrition, chronic renal failure, liver disease [including cirrhosis], cancer, acquired immunodeficiency syndrome)
Radiation treatment of head and neck
Congenital conditions (e.g., congenital anosmia, Kallmann's syndrome)

Uncommon causes
Neoplasm or brain tumor (e.g., osteoma, olfactory groove or cribiform plate meningioma, frontal lobe tumor, temporal lobe tumor, pituitary tumor, aneurysm, esthesioneuroblastoma, melanoma, squamous cell carcinoma)
Psychiatric conditions (e.g., malingering, schizophrenia, depression, olfactory reference syndrome)
Endocrine disorders (e.g., adrenocortical insufficiency, Cushing's syndrome, diabetes mellitus, hypothyroidism, primary amenorrhea, pseudohypoparathyroidism, Kallmann's syndrome, Turner's syndrome, pregnancy)
Epilepsy (olfactory aura)
Migraine headache (olfactory aura)
Cerebrovascular accident
Sjögren's syndrome
Systemic lupus erythematosus