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How Does Plan B Work?

Plan B One-Step™ (levonorgestrel) is an emergency contraceptive used to prevent pregnancy in cases of unprotected sex. But how does Plan B work?
This contraceptive contains a high dose of levonorgestrel, a progesterone-type hormone that is used in many birth control pills. Plan B works to prevent 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.
This medication 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.
Plan B was originally a prescription medication. However, it is now available without a prescription to women of all ages.
(For more information on how Plan B works, click Plan B. This article provides a complete overview of this medicine, including information on its uses, general precautions, and potential side effects.)
Last reviewed by: Kristi Monson, PharmD
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