The sooner you take Plan B®, the more effective it is. It can prevent pregnancy if taken within 72 hours and preferably within 12 hours of unprotected sex. If you take it within 24 hours of unprotected sex, it is 95% effective. If you take it between 48 and 72 hours of unprotected sex, the efficacy rate is 61%.

How long does it take for Plan B to kick in?

Plan B works if taken within 72 hours after a regular birth control method fails or within 72 hours after unprotected intercourse. Once absorbed into the bloodstream, which usually takes a couple of hours, levonorgestrel starts to affect the ovaries or uterine lining.

How do I know if Plan B worked?

How do I know if Plan B worked? You'll know if your emergency contraception worked when you get your next period, which could come within a week of when you expect. Your period may be heavier, lighter, earlier, or later than usual.