The FDA has said as much since the first morning-after pill was approved in 1998. The Plan B brand pill, which was under FDA scrutiny, contains a higher dose of one of the same hormones found in regular birth control pills. The pill is most effective if taken within 24 hours of intercourse, although it's still effective for up to 72 hours. In some instances, it delays ovulation and prevents fertilization of an egg. In other instances, it prevents the implantation of a fertilized egg in the uterus.