How well does emergency contraception work to prevent pregnancy?

89 %: Plan b or emergency hormonal contraception is almost 90% effective if it is used within 72 hours of intercourse, and there has only been one ejaculation in that time frame.

It can be very: effective depending on when it is taken. Here is an example > Plan B One-Step is up to 89 % effective when taken within 72 hours (3 days) after unprotected sex. It can reduce chance of pregnancy by up to 95% if used within 24 hours. It continues to reduce the risk of pregnancy up to 120 hours (5 days) after unprotected sex, but is less effective as time passes.