How does emergency contraception work?

Several: It may prevent or delay ovulation, interfere with fertilization, or prevent implantation.

ECP's may: delay release of an egg from an ovary, prevent fertilization or prevent a fertilized egg from attaching to the uterus.

Depends how soon: Depends when you take it. The sooner after unprotected sex the better it works. Varies from around 95% effective if taken right away to about 85% if taken 3-5 days later. Consider a copper iud if you're looking for something more effective.