The effectiveness of plan B will depend on when it is taken. If you take it within 72 hours after you've had unprotected sex, Plan B can reduce the risk of pregnancy by up to 89%. If you take Plan B within 24 hours, it is about 95% effective. Go to http://women.webmd.com/guide/plan-b for details.
When used properly, in the prescribed time frame, Plan B is 89% effective. Keep in mind it *must* be used within 3 days of unprotected sex and it *will not* terminate a pregnancy if the woman is already pregnant.