How long after sex does emergency contraception work?

72 hours: Plan b - the emergency contraception pill - is indicated to use up to 72 hours after unprotected sex. The earlier you take it the better your chances of preventing pregnancy. I often recommend patients to keep it on hand so they can take it as early as possible after exposure.

ECP: Emergency Contraception like 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.