Now, the side effects. First, Plan B can “potentially alter when your next cycle would be,” Strachan explains. Your period may arrive up to a week earlier or later than usual, or it could not be affected at all. It also may be lighter or heavier than usual. Additionally, you may see some spotting between the time you take Plan B and your next period. Generally, your period should return to normal by your next cycle, but if you take Plan B often , your period could become irregular for a longer time. That's part of why Plan B is a type of emergency contraception — it's meant to help prevent pregnancy if your regular form of contraception falls through, not serve as your primary form of birth control