Emergency contraceptive pills (ECPs) work before pregnancy. They must be taken as soon as possible after unprotected sex, and are recommended to be taken within 3-5 days. They are not effective after this time. They work by preventing the release of an egg (ovulation) or by stopping the egg and sperm from meeting. The use of ECPs cannot terminate or interrupt an established pregnancy and will not stop a fertilized egg from implanting in the uterus, nor do they harm a developing embryo.
This video on how EC works clearly explains EC’s mechanism of action.
The most common type of ECP is levonorgestrel-only, or LNG ECPs. Extensive research has been conducted on how LNG ECPs work and there is clear evidence that interference with ovulation is the primary mechanism of action. It is possible that levonorgestrel may interfere with other events prior to fertilization (such as impairing the migration of sperm), but it does not have effects after fertilization. No evidence supports the theory that LNG ECPs interfere with the implantation of a fertilized egg. For more information, see our fact sheet LNG ECPs’ mechanism of action. To learn more about the difference between emergency contraception and medical abortion, click here.