The injection form of birth control, Depo-Provera, has become popular, especially among teens. It's a progesterone-only hormone: there's no estrogen in the preparation. It works by suppressing the lutenizing and follicle -stimulating hormones to prevent ovulation. It usually stops menstruation, too.

Depo is injected into the upper arm or the buttocks every three months. It's very effective, with a 1% failure rate, provided you stick to the schedule.

Depo-Provera is an excellent choice for females who are not reliable about taking the birth control pill. It is also beneficial for women with congenital heart disease and women who have heavy menstrual periods.

In the first year of Depo use, about one-third of patients experience some breakthrough bleeding. After that, only 10% of patients are affected. Generally, periods stop, which for young women is a real benefit, as long as you haven't been relying on your period to confirm that you're not pregnant.

Potential negative side effects include depression, weight gain, severe headaches, tender breasts and/or bloating. Depo does not increase the risk of breast cancer. It has now been proven that the drug prevents the absorption of calcium into the hip bones, spine and the long leg bones. For teenage females, this is the time of their life when they should be building up calcium. As with the pill, smoking is dangerous.

Females considering getting pregnant should know that, when they stop the injections, it may take 9 months to 1 year before ovulation and regular menstrual periods begin again.

Depo costs about the same as taking the pill. But with Depo, "you get it and forget it" for 3 months.