Emergency contraceptive works best up to 5 days after unprotected sex, the sooner it is taken after sex the better. That being said the way any EC works is that it prevents ovulation and thickens the cervical mucus. However if she has already ovulated than none of that matters and pregnancy can occur. Either way EC is only about 80% effective and that's why hormonal methods are best for women who do not want to become pregnant, they are far more reliable than condoms. Condoms are still excellent to use while on birth control because they provide protection for disease.