Contraceptive Injections

Two contraceptive formulations are available as injections. Each is injectedby a health care practitioner into a muscle of the arm or buttocks, and eachis very effective as a contraceptive.

Medroxyprogesterone acetate, a progestin, is injected once every 3 months.Medroxyprogesterone acetate can completely disrupt the menstrual cycle.About one third of women using this contraceptive have no menstrual bleedingduring the 3 months after the first injection, and another third haveirregular bleeding and spotting for more than 11 days each month. After thiscontraceptive is used for a while, irregular bleeding occurs less often.

After 2 years, about 70% of the women have no bleeding at all. When theinjections are discontinued, a regular menstrual cycle resumes in about halfof the women within 6 months and in about three fourths within 1 year.

Fertility may not return for up to a year after injections are discontinued.Side effects include a slight weight gain and a temporary decrease in bonedensity. Bones usually return to their previous density after the injectionsare discontinued. Medroxyprogesterone acetate does not increase the risk ofdeveloping any cancer, including breast cancer. It greatly reduces the riskof developing uterine (endometrial) cancer. Interactions with other drugsare uncommon.

The other formulation is a once-a-month injection. It contains estrogen anda much smaller amount of medroxyprogesterone acetate than the injectionsgiven every 3 months. Consequently, bleeding usually occurs regularly about2 weeks after each injection is given, and bone density does not decrease.Because the dose of medroxyprogesterone acetate is lower, fertility returnsmuch more rapidly after the injections are discontinued. [Merck]