How effective is plan b if taken 12 hours after sexual intercourse?

95%: The sooner its taken the higher the efficacy. Under 24 hours -95% effective, within 72 hours, 89% effective.

Plan B: Plan B One-Step can reduce your risk of pregnancy by up to 95% if used within 24 hours of sex. If taken within 72 hours it can reduce risk of pregnancy by up to 89%. But you should know that Plan B One-Step is not as effective as regular contraception. Please use protection every time to prevent pregnancy and sexually transmitted disease.