Plan B is emergency contraception.
Plan B can reduce a woman’s risk of pregnancy when taken as directed if she has had unprotected sex. Plan B contains only progestin (levonorgestrel) a synthetic hormone used in birth control pills for over 35 years. In the United States, it is currently available only by prescription. Plan B works like other birth control pills to prevent pregnancy.
Plan B acts primarily by stopping the release of an egg from the ovary (ovulation). It may prevent the union of sperm and egg (fertilization). If fertilization does occur, Plan B may prevent a fertilized egg from attaching to the womb (implantation).
Plan B should be taken within 72 hours. If a fertilized egg is implanted prior to taking Plan B, Plan B will not work. http://www.fda.gov/cder/drug/infopag...planBQandA.htm