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How do drugs cause hair loss?

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Drugs cause hair loss by interfering with the normal cycle of scalp hair growth. During the anagen phase, which lasts for 2 to 6 years, the hair grows. During the telogen phase, which lasts about 3 months, the hair rests. At the end of the telogen phase, the hair falls out and is replaced by new hair.

Medications can lead to two types of hair loss: telogen effluvium and anagen effluvium.

Telogen effluvium is the most common form of drug-induced hair loss. It usually appears within 2 to 4 months after taking the drug. This condition causes the hair follicles to go into their resting phase (telogen) and fall out too early. People with telogen effluvium usually shed between 30% to 70% more than the normal 100 and 150 hairs a day.

Anagen effluvium is hair loss that occurs during the anagen phase of the hair cycle, when the hairs are actively growing. It prevents the matrix cells, which produce new hairs, from dividing normally. This type of hair loss usually occurs within a few days to weeks after taking the medication. It's most common in people who are taking chemotherapy drugs for cancer and is often severe, causing people to lose most or all of the hair on their head, as well as their eyebrows, eyelashes, and other body hairs.

The severity of drug-induced hair loss depends on the type of drug and dosage, as well as your sensitivity to that drug.

SOURCES:

Bolognia, J.L., Jorizzo, J.L., Rapini, R.P., eds, , 2nd ed., Philadelphia, Mosby Elsevier, 2008. Dermatology

Tosti, A. , 2007. Dermatologic Clinics

Mounsey, A.L. , 2009. American Family Physician

Tosti A. , 1994. Drug Safety

National Guideline Clearinghouse: "Recommendations to diagnose and treat adult hair loss disorders or alopecia in primary care settings (non pregnant female and male adults)."

American Cancer Society web site.

Reviewed by Stephanie S. Gardner on November 16, 2018

SOURCES:

Bolognia, J.L., Jorizzo, J.L., Rapini, R.P., eds, , 2nd ed., Philadelphia, Mosby Elsevier, 2008. Dermatology

Tosti, A. , 2007. Dermatologic Clinics

Mounsey, A.L. , 2009. American Family Physician

Tosti A. , 1994. Drug Safety

National Guideline Clearinghouse: "Recommendations to diagnose and treat adult hair loss disorders or alopecia in primary care settings (non pregnant female and male adults)."

American Cancer Society web site.

Reviewed by Stephanie S. Gardner on November 16, 2018

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What types of drugs cause hair loss?

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