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Plan B Effectiveness

Plan B One-Step™ (levonorgestrel) is an emergency contraceptive used within 72 hours of unprotected sex to help prevent pregnancy. Taking it within 72 hours after unprotected sex reduces the risk of pregnancy by 89 percent. After any one act of unprotected intercourse, the average woman has an 8 percent chance of getting pregnant. Studies on the effectiveness of Plan B show that if she takes drug, that risk is reduced to about 1 percent.
Plan B contains a high dose of levonorgestrel, a progesterone-type hormone that is used in many birth control pills. It effectively prevents pregnancy by stopping ovulation (the maturation and release of eggs from the ovaries) or by preventing fertilization of the egg if ovulation has already occurred. It also alters the lining of the uterus (the endometrium), making it less receptive to a fertilized egg.
If a fertilized egg has already implanted into the uterus, Plan B will not cause an abortion.
(For more information on Plan B's effectiveness, click Plan B. This article also discusses potential side effects, general safety concerns, and what to be aware of before using this emergency contraceptive.)
Last reviewed by: Kristi Monson, PharmD
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