Hormones, including birth control pills, can affect the hair cycle, sometimes throwing out of balance the normal proportion of growing hairs to hairs that are falling out.

About 90% of hairs are normally in the growing (anagen) phase at any one time, while about 10% are in a resting (telogen) phase. A hair in telogen phase falls out and a new hair begins to grow in its place. Thus the cycle of growth and shedding followed by new growth continues in each hair follicle. Normal shedding is estimated at approximately 100 hairs per day.

Each hair follicle has its own cycle of growth, shedding and regrowth. If all hairs were in the same cycle, we would grow a full head of hair and then shed it all, then regrow a full head of hair again. Instead, every hair follicle cycles on its own, and is not synchronized with other hair follicles. In this way the process of growing and shedding can occur while we still maintain hair on our heads.

Many factors can alter the normal balance of hair growth to hair shedding, including physical stress, illness, and medications such as cholesterol-lowering drugs, antidepressants, Vitamin A derivatives, and certain high blood pressure medications (beta blockers).

Hormones play a role in stimulating hair follicles to grow hairs. Hormones, including birth control pills, can also affect the balance, causing more growth at some times and shedding at other times. Since there are many other causes of hair loss, it is important to see your doctor to determine if there are other factors or scalp conditions that may be contributing to your hair loss.