Plan B works by temporarily delaying ovulation, preventing sperm from fertilizing an egg
. The sooner it is taken after unprotected sex, the more effective it is
. Plan B delays ovulation for approximately 5 days, but can range from a few days up to 5 to 7 days, depending on where you are in your cycle
. It is most effective when taken within 72 hours of unprotected sex
. However, if ovulation has already begun, Plan B will not be effective
