The morning after pill works primarily by preventing or delaying ovulation, which is the release of an egg from the ovary. Without an egg released, sperm cannot fertilize it, thus preventing pregnancy. Additionally, the hormones in the pill can make the lining of the womb less suitable for an egg to attach if fertilization has occurred, but it does not terminate an existing pregnancy. There are two main types of morning after pills:

• Levonorgestrel pills, which can be taken up to 72 hours (3 days) after unprotected sex and work best the sooner they are taken.
• Ulipristal acetate pills, which can be effective up to 120 hours (5 days) after unprotected sex and tend to be more effective than levonorgestrel pills.

The morning after pill is an emergency contraceptive meant to be used after unprotected sex or contraceptive failure, and it is not intended as a regular birth control method. It does not protect against sexually transmitted infections or end an existing pregnancy.