Common side effects of Plan B include:
abdominal or stomach pain,
changes in menstrual periods,
breast pain or tenderness,
What does Plan B do to you?
plan B® comes in a one-pill dose. If taken within 72 hours (3 days) and preferably within 12 hours after a contraceptive accident or unprotected sex, it can prevent pregnancy by doing one of three things: Temporarily stops the release of an egg from the ovary. Prevents fertilization.