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.

Can Plan B mess up your period?

According to the package insert, Plan B can cause other changes to your period, such as heavier or lighter bleeding or getting your period earlier or later than normal. This type of bleeding after taking Plan B isn't generally a cause for concern.

Can Plan B delay your period for a month?

The number one side effect of taking Plan B is irregular periods, which can come early or be delayed by a couple of days. Taking emergency contraception could also affect your next month's period. But the effects usually resolve on their own, and no treatment is necessary.