Plan B® (levonorgestrel) is an emergency contraceptive used after unprotected sex. Although this medication can help prevent a pregnancy from occurring within 72 hours after unprotected sex, it will not cause an abortion if a fertilized egg has already implanted into the uterus.
As a contraceptive, Plan B prevents pregnancy primarily 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.
Taking Plan B birth control pills within 72 hours after unprotected sex reduces the risk of pregnancy by 89 percent.
(Click Plan B for a complete overview of this contraceptive, including information on how this medication works, potential side effects, general dosing guidelines, and what to tell your healthcare provider before using it.)
