If you are using the plan b emergency contraceptive, when should you take the two pills?

Within 72 hours: You must use ir within 72 hours of the episode of unprotected intercourse. Even then it is only 75% effective so you must take a pregnancy test if you are late with your period.

As soon as possible: after sexual intercourse. 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.