Sometimes referred to as “the morning-after pill,” Plan B is an over-the-counter medication used to prevent pregnancy within 72 hours of having unprotected sex. Plan B works in three ways. First, the medication prevents ovulation so an egg will not be released. If the egg has already been released, Plan B will prevent the egg from being fertilized by the sperm. If an egg has already been fertilized, Plan B’s last defense against pregnancy is to alter the uterine lining to prevent implantation.