Medications may cause hair loss by a variety of mechanisms including anagen arrest, telogen effluvium, or in the case of exogenous androgens, by accentuation of androgenetic alopecia.

Drug-induced cicatricial or scarring alopecia is uncommon.

Hair loss, and in particular telogen effluvium, may also occur in response to a number of triggers including fever, hemorrhage, severe illness, stress, and childbirth, and a thorough exclusion of these potential confounders is necessary before the hair loss can be attributed to the medication. In addition, androgenetic alopecia is commonly punctuated by an episode of increased hair shedding that lasts 2 to 4 months, and this shedding may further confuse the clinical picture.

Many reports in the literature that attribute hair loss to particular medications have not adequately explored the nature of the hair loss and excluded other potential unrelated causes of hair loss; therefore, they are difficult to interpret.

Anagen effluvium is a form of diffuse hair loss that follows administration of anticancer chemotherapeutic agents, radiation treatment, and various chemicals. The degree of hair loss is dependent on the route, dose and schedule of the chemotherapy agent.

Drug-induced telogen effluvium mainly involves premature interruption of hair growth with an early entry of anagen follicles into the resting phase, leading to a noticeable increase in hair shedding 2 to 3 months later.

In almost all cases, there is recovery of hair loss within 3 months following discontinuation of the medication. Many medications have been suspected of causing hair loss. A high index of suspicion should be maintained regarding medication-induced alopecia, especially if other causes have been excluded.