Plan B One-Step, the latest version of the drug as of 2014, is 95 percent effective if taken within 24 hours of unprotected sex, explains WebMD. The Plan B One-Step official website claims that the pill prevents pregnancy in seven out of eight women who would have otherwise become pregnant.
Plan B One-Step can be taken at any time in the 72 hours after unprotected sex, but the efficacy rate drops slightly after the first 24 hours. WebMD explains that the pill has an 89 percent effectiveness rate if taken after 24 hours have passed but before 72 hours are up. The Plan B One-Step website warns that Plan B One-Step is not intended for regular use as a pregnancy prevention method and should only be taken in emergencies.