How effective is an emergancy contraceptive pill like plan b? how fast do i need to take it for the maximum effect?

Take it as soon as: possible after the episode with failed birth control. 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.