What are the possible causes of anosmia?

Smell disorders are often classified as one of the following:
•    anosmia: complete loss of the ability to detect odors
•    hyposmia: decreased sense of smell with some ability to detect odors
•    dysosmia: distorted sense of smell.
Hyposmia commonly happens as we age. Other causes of loss of smell or distorted smell include:
•    COVID-19 or a cold or sinus infection
•    hay fever (allergic rhinitis)
•    nasal polyps
•    a medication, such as the cholesterol-lowering drug atorvastatin (Lipitor), the blood pressure drug amlodipine or the antibiotic erythromycin
•    a side effect of general anesthesia.