How Does Hair Grow?
Learning how hair grows is important to understanding how hair loss occurs and what can be done to treat it. Hair is made of keratin that is produced in the papilla which is located within the hair follicle under the skin. The part of the hair that rises out of the follicle or the visible part of hair is called the shaft. Hair growth is caused by actively dividing cells within the follicle. As a result hair growth requires a healthy scalp and good blood flow to the follicle.
Hair growth is divided into two phases; anagen or the active phase and telogen the resting phase. During anagen, the hair shafts grow out of the follicles. This phase lasts for about five years. During this phase, cells are rapidly dividing. After the growth phase the hair goes into the telogen phase. This phase lasts about 12 weeks. After this resting period the hair falls out as new hair shafts start growing from the follicle, displacing the resting shaft. A human head has about 120, 000 hair on the scalp and loses about 100 hair shafts a day. This normal hair loss is not obvious; however, in alopecia there is a noticeable difference.
Most causes of hair loss are hereditary or as a result of hormonal changes. However, hair loss can also occur as a result of trauma, disease, and chemical or medication interference.
How Drug Induced Alopecia Occurs
Drug induced hair loss occurs as a result of chemical interference to the natural growth cycle. The magnitude, the effect rate, and the duration of the hair loss depend on the phase when the interference occurred. If drug exposure occurs during the telogen phase, hair loss is typically noticeable between one and six months after exposure. If the drugs damage cell activity during the anagen phase, then hair loss is noticeable within a couple of weeks.
Chemotherapy drugs target rapidly dividing cancer cells. These drugs are cytotoxic causing the death of their target cells. However, chemotherapy drugs do not discriminate; they target ALL rapidly dividing cells which include the rapidly dividing cells within the hair follicle.
The drugs either disrupt the new hair growth by interrupting it or arresting it. Interruption leaves only fine underdeveloped hair. Drugs can also completely arrest the cell processes, stopping hair growth completely and causing all the hair to fall out.
Medications that Cause Hair Loss
- Chemotherapeutic Drugs: Most commonly known drugs that cause hair loss are drugs that are used to treat rapidly dividing cancerous cell. Since, hair cells divide rapidly they are also affected. However, not all chemotherapy drugs cause hair loss; they may cause hair thinning, but not loss. However, the combination of drugs may cause hair loss. Chemotherapy drugs associated with hair loss include,
- Prescription drugs associated with hair loss: bismuth, levodopa (used to treat Parkinson’s disease), cyclosporine (immunosuppressant used to prevent organ rejection), and colchicines (used for treating gout).
- Other medications: beta-blockers, carbamazepine (used to manage epilepsy), anticoagulants, and retinoids.
The good news is that drug induced hair loss is reversible. As soon as the medication is eliminated, then the normal growth cycle is resumed.
- Robert A Schwartz; Anagen Effluvium: eMedicine Dermatology, September 2009
- Elizabeth Hughes; Telogen Effluvium; eMedicine Dermatology, March 2009