Anagen effluvium is a form of nonscarring alopecia with abrupt loss of anagen hair (hair that is in the growing phase), resulting in excessive hair loss. This is triggered by an event that impairs the mitotic and metabolic potential of anagen hair, such as chemotherapy treatment, radiation therapy, and some inflammatory diseases. At any given time, approximately 90% of normal scalp is in the anagen phase, resulting in hair breakage and volume loss. In anagen effluvium, the proliferating cells in the hair bulb are arrested in anagen phase, while the stem cells of the bulge area are spared. Hence hair loss is usually reversible. Up to 80%-90% of hair loss can occur. The interval between the inciting event or exposure and the hair shedding is generally 1-4 weeks, as opposed to weeks to months in telogen effluvium.

Anagen effluvium can occur in up to 65% of patients undergoing chemotherapy. Drugs implicated include antimicrotubule agents (eg, paclitaxel), topoisomerase inhibitors (eg, doxorubicin), alkylating agents (eg, cyclophosphamide), and antimetabolites (eg, 5-fluorouracil). Other medications such as bismuth, levodopa, colchicine, and cyclosporine, and exposure to toxic chemicals such as thallium, mercury, boron, copper, or cadmium, have also been implicated. Although drug-induced alopecia is usually reversible upon discontinuation of the culprit medication, permanent alopecia from chemotherapy (taxane and adjuvant hormonal therapy) has been reported.

Other causes of anagen effluvium include protein malnutrition, pemphigus vulgaris, systemic lupus erythematosus, alopecia areata, and secondary syphilis. Finally, radiation therapy is a known trigger and may result in irreversible alopecia (radiation-induced alopecia).

Anagen effluvium is self-limiting, and the duration of disease is dependent on the duration of toxic insult.
High dosages of drug therapy and polypharmacy might be associated with a more severe phenotype.