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.