Long-acting, injectable contraceptives first became available in the 1960s. It is currently estimated that almost 3.5 million women are now using depo-medroxyprogesterone acetate (DMPA); 800,000 are using norethindrone enanthate (NET-EN), and another few hundred thousand are using a variety of once-a-month injectables comprised of progestin plus estrogen. The advantages of injectable contraceptives are that they are highly effective, independent of coitus, easily administered, and they ensure regular contact with health services personnel. The last factor may be considered a disadvantage by some, since contact is more frequent than would be required for routine health services. The major disadvantage of the progestin-only formulations is disruption of normal menses, giving rise to unpredicted episodes of bleeding and spotting. With the once-a-month formulation, on the other hand, there are few discontinuations due to disruption of menses. For a long-acting method to be used longer than 6 months, it is desirable to choose an implant, since the method can be discontinued at will. The first implant system to be developed was Norplant, a set of six rubber capsules filled with levonorgestrel and implanted under the skin. The implant releases sufficient levels of medication to protect against pregnancy. For the first 5 years, the average failure rate was four or five per thousand users per year. The failure rate for women using standard oral contraceptives is approximately 20 to 50 per thousand. The most common side effect of the implant method is the disruption of the menstrual cycle, an effect that is particularly marked in the first month of use.(ABSTRACT TRUNCATED AT 250 WORDS)