Cause	Leading etiologies*
Most common
Sinonasal conditions	Upper respiratory infection (especially viral), allergic rhinitis, chronic rhinosinusitis, nasal polyps
More common
Head trauma	Damage to cribriform plate, shearing forces, and intracranial damage; facial trauma
Neurodegenerative disorders	Parkinson disease, parkinsonism, Alzheimer disease, mild cognitive impairment, multiple sclerosis
Less common
Medications	Chemotherapy, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, dihydropyridine calcium channel blockers, diuretics, intranasal zinc, antimicrobials (macrolides, terbinafine [Lamisil], fluoroquinolones, protease inhibitors, griseofulvin, penicillins, tetracyclines, nitroimidazoles [metronidazole (Flagyl)]), antiarrhythmics, antithyroid agents, antidepressants, anticonvulsants, lipid-lowering agents
Intoxicants or illicit substances	Alcohol, cocaine
Toxins	Ammonia, hairdressing chemicals, gasoline, formaldehyde, paint solvents, welding agents, benzene, sulfuric acids, cadmium, acrylates, iron, lead, chromium
Chronic medical conditions	Renal or hepatic failure, complicated type 2 diabetes mellitus, cancer, human immunodeficiency virus
Structural or mechanical conditions	Ischemic stroke, subarachnoid or intracranial hemorrhage, brain or sinonasal tumor
Nutritional deficiencies	Malnutrition, pernicious anemia or vitamin B12 deficiency, deficiencies in vitamins B6 or A, niacin, zinc, or copper
Postsurgical state	Nasal surgery (septal or sinus), total laryngectomy, pharyngectomy, tonsillectomy
Post-radiation	Especially to head and neck
Congenital conditions	Kallmann syndrome, anosmia
Psychiatric conditions	Anorexia nervosa (not bulimia), major depressive disorder, bipolar disorder, schizophrenia
Endocrine conditions	Pregnancy, hypothyroidism, Addison disease, Cushing syndrome
Autoimmune/inflammatory conditions	Sjögren syndrome, systemic lupus erythematosus, sarcoidosis, herpes encephalitis