Plan B is a form of emergency contraception. It is usually sold under the name "Plan B One Step." Effective in preventing pregnancy when taken up to 72 hours after unprotected sexual intercourse, Plan B works in one of two ways, depending on when in the menstrual cycle it is ingested. It can either prevent ovulation or interfere with fertilization. It is also possible that Plan B affects the uterine lining works to prevent implantation.