Yes, Plan B can make your period late. It can also cause your next period to come earlier or be slightly different than usual. These changes are common and usually temporary.
Plan B contains levonorgestrel, a hormone that can affect your normal menstrual cycle. This can shift the timing of ovulation, which then changes when your period starts. Because of this, your next period may come a few days late, or sometimes up to about a week later than expected.
It is also normal to notice spotting or irregular bleeding in the days or weeks after taking it. In most cases, your cycle returns to normal by the following month. These effects are not harmful and do not mean the medication did not work.
If your period is more than a week late, taking a pregnancy test is recommended. If you have ongoing irregular cycles or concerns, speak with a gynecologist or primary care provider, and avoid taking additional medication without guidance.