Emergency Contraceptive Pills

Modern emergency contraception began with the Yuzpe regimen in the 1970s. This regimen consists of 100 ^.g ethinyl estradiol and 0.5 mg of levonorgestrel or 1 mg norgestrel taken within 72 hours of unprotected intercourse, followed by a repeat dose 12 hours later. This was initially accomplished by taking high doses of COCs, but began to be marketed as a specific product, Preven, in 1998 (Stewart et al., 2007). This method has a failure/pregnancy rate of 2-3%, with an average 74% reduction in the number of expected pregnancies when used (Trussel et al., 1996). Side effects are common, with about 50% of women experiencing nausea and 20% vomiting from the high doses of estrogen. Subsequent research has led to the preferred use of other EC methods, and Preven was pulled from the market in 2004.

A 1998 study by the WHO Task Force on Postovulatory Methods of Fertility Regulation compared two doses of 0.75 mg of levonorgestrel given 12 hours apart to the Yuzpe regimen. The levonorgestrel-only method was both better tolerated and more effective than the Yuzpe regimen, with a failure/pregnancy rate of 1.1% and an 85% reduction in the number of expected pregnancies. In 1999 the FDA approved Plan B, which contains two 0.75-mg tablets of levonorgestrel. A single 1.5-mg dose of levonorgestrel taken up to 5 days after unprotected intercourse seems to be equally effective at pregnancy prevention (von Hertzen et al, 2002) (Level A). Plan B One-Step, a single pill containing 1.5 mg of levonorgestrel, is currently available.

Mechanism of Action

Treatment with levonorgestrel before ovulation inhibits the LH surge and therefore ovulation. If taken after ovulation, ECPs have little effect on hormone production and only a limited effect on the endometrium (Marions et al., 2002). Therefore, levonorgestrel for emergency contraception works by inhibiting ovulation; it is not an abortifacient.

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