Women can use emergency contraception to prevent pregnancy after known or suspected failure of birth control or after unprotected intercourse. Many patients do not ask for emergency contraception because they do not know of its availability. Emergency contraception has been an off-label use of oral contraceptive pills since the 1960s. Dedicated products, the Yuzpe regimen (Preven) and levonorgestrel (Plan B), were marketed in the United States after 1998 but had been available in Europe for years before that. A third approved method of emergency contraception is the insertion of an intrauterine device. Emergency contraception is about 75 to 85 percent effective. It is most effective when initiated within 72 hours after unprotected intercourse. The mechanism of action may vary, depending on the day of the menstrual cycle on which treatment is started. Despite the large number of women who have received emergency contraception, there have been no reports of major adverse outcomes. If a woman becomes pregnant after using emergency contraception, she may be reassured about the lack of negative effects emergency contraception has on fetal development. It may be beneficial for physicians to offer an advance prescription for emergency contraception at a patient's regular gynecologic visit to help reduce unwanted pregnancies. Advance provision of emergency contraception can increase its use significantly without adversely affecting the use of routine contraception.