The most effective morning after pill depends on when you take it. According to Planned Parenthood, the morning after pill is up to 89% effective in preventing pregnancy when taken within 72 hours of unprotected sex. But, the effectiveness drops to about 75% if taken within 72-120 hours of unprotected sex. It is essential to understand that the morning after pill does not work if you are already pregnant. It is also not a form of regular birth control and should not be taken as a replacement for contraceptives.