Levonorgestrel is found in birth control pills, but Plan B contains a higher dose that can alter your body's natural hormone levels. The extra hormones can, in turn, affect the menstrual cycle, leading to an earlier or delayed period as well as heavier or lighter bleeding.

How late can your period be after taking Plan B?

Your cycle should also go back to normal soon. Most of the changes caused by emergency contraceptives like Plan B do not persist beyond one menstrual cycle. If your period is over a week late after you've taken Plan B, there's a chance that you may be pregnant, so consider taking a pregnancy test.

What happens when you take Plan B on your period?

It causes a change in hormone levels, which can affect the usual pattern of your menstrual cycle. This can lead to spotting between the time you take it and the beginning of your next period. It can also cause your period to start up to a week earlier or a week later than you would otherwise expect.