Progestin-only pills, also called the 'minipill', are used in a manner similar to combined OCs. The progestin-only minipill has a dose of progestin that is very close to the threshold of contraceptive efficacy. Therefore these pills must be taken at approximately the same time each day and are taken continuously for 28 days of the month without a pill-free interval. Less than one percent of oral contraceptive prescriptions in the United States are for the progestin-only oral contraceptive (88). This form of contraception is traditionally most often used in women who are breastfeeding or in women who have contraindications to estrogen, however, most women are candidates for this method. Since the 1970's, two progestin-only formulations are available in the United States, one that contains 75mcg of norgestrel and the other, 350mcg of norethindrone. It is important to note that ovulation is not always inhibited with the use of progestin-only pills. Approximately half of cycles have suppressed ovulation and thus contraceptive efficacy is dependant on the other progestin related mechanisms listed previously. (88)

Efficacy: The typical failure rate of progestin-only pills is similar to that with combined oral contraceptives (89), despite the fact that efficacy is only for 27 hours and requires consistent administration. Serum levels of progestin peak 2 hours after administration and return to near baseline levels within 24 hours (88). Variation of only a few hours in administration can be the difference in the progestin-only pill providing its contraceptive protection. Women should be prepared to use a back-up method if they are three hours late in taking the pill, if one pill is missed or if there is a delay in its administration.

Side Effects: The main side effect associated with progestin-only pills is menstrual cycle irregularity. Spotting or breakthrough bleeding, amenorrhea, and shortened length of menstrual cycles are the most common irregularities experienced. A randomized, double-blind study by the WHO showed that an average of 53% of users had frequent bleeding, 22% had prolonged bleeding, 13% had irregular bleeding, and 6% had amenorrhea within 3 months of initiation (90). Menstrual irregularity is a common reason for method discontinuation. Other less common side effects include nausea, dizziness, headache, and breast tenderness.

Risks: In general, any contraceptive method protects against ectopic pregnancy. However, if progestin-only pill users get pregnant, on average 6-10% of the pregnancies will be ectopic, higher than the rate seen in women not using any method of contraception (2%) (88). The overall risk, however, remains lower than the general population because so few women actually become pregnant (7%) while using this method of contraception.

A recent WHO case-control study of cardiovascular disease and progestin-only pill use found no significant increase in the risk of acute myocardial infarction (RR=1.0, 95%CI, 0.2-6.0), stroke (RR=1.1, 95%CI, 0.6-1.9), or venous thromboembolism (RR=1.8, 95%CI, 0.8-4.2) (91). Thus far, progestin-only pills appear to have little or no effect on lipid metabolism, carbohydrate metabolism, hypertension, and coagulation factors (88).