Contraceptive Injection

Contraceptive Injection, drug containing one or more female sex hormones that is injected into the muscles of a woman’s arm or buttocks to prevent pregnancy. These hormones halt ovulation, the monthly release of an egg from the ovary. They also thicken the mucus of the cervix (opening of the uterus), making it difficult for sperm to pass into the uterus. In addition, the hormones may prevent the lining of the uterus from thickening so that a fertilized egg cannot implant. Contraceptive injections are one of the most effective types of reversible birth control; with typical use they prevent pregnancy 99 percent of the time.

In the United States, two types of contraceptive injections are available: Depo-Provera and Lunelle. Depo-Provera, available since 1992, contains the synthetic hormone progestin and must be injected every 12 weeks. Lunelle, which became available in 2000, contains a combination of estrogen and progestin and must be injected every month. A health-care professional must administer the injections.

A woman who receives contraceptive injections does not need other birth control methods to prevent pregnancy. Many women prefer this method because it does not interfere with sexual spontaneity. Depo-Provera is particularly useful for those women who have medical conditions, such as a history of blood clots, that prevent them from using contraceptives that contain estrogen. Studies show that contraceptive injections decrease the risk of endometrial cancer (cancer of the uterine lining).

The most common side effect of a contraceptive injection is irregular bleeding for the first few months of use. About half of all women who use this contraceptive stop menstruating (a condition known as amenorrhea), sometimes for a year or more. This side effect has not been shown to cause health problems. Other possible side effects include weight gain, headaches, mood changes, and abdominal pain. In many cases, side effects clear up as a woman continues to use the contraceptive. Contraceptive injections do not protect against sexually transmitted infections (STIs).