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 the 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 the 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. Cancer treatment-induced alopecia depends on the specific agent and the route, dose, and schedule of administration. High-dose, intermittent, intravenous (IV) chemotherapy is more likely to cause higher-grade alopecia than low-dose therapy, oral administration, and weekly IV regimens. Combination therapies are more likely to cause some level of alopecia compared to single agents. Chemotherapy agents at highest risk of causing alopecia include cytotoxic agents (ie, alkylating agents, antitumor antibiotics, antimicrotubule agents, and topoisomerase inhibitors), endocrine therapy (ie, estrogen antagonists, aromatase inhibitors, CDK4/6 inhibitors), and molecularly targeted agents (ie, EGFR inhibitors). Other risk factors include poor drug metabolism, prior exposure to scalp irradiation, older age, presence of androgenic alopecia, prior chemotherapy-induced alopecia, and graft-versus-host disease.

Other causes of anagen effluvium include protein malnutrition, pemphigus vulgaris, systemic lupus erythematosus, alopecia areata, and secondary syphilis. 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.

The most common cause of drug-induced alopecia in children is chemotherapy-induced anagen effluvium.

Thiotepa is the drug most commonly involved in permanent chemotherapy-induced alopecia.