From the FDA Drug Label
some women may have changes in their period, such as a period that is heavier or lighter or a period that is early or late. If your period is more than a week late, you may be pregnant You will know levonorgestrel tablet has been effective when you get your next period, which should come at the expected time, or within a week of the expected time.
Your menstrual cycle may be irregular after taking Plan B, with changes in your period, such as a period that is heavier or lighter or a period that is early or late. You can expect your next period to come at the expected time, or within a week of the expected time 1.
From the Research
Plan B (levonorgestrel) typically causes menstrual irregularities for about one cycle after taking it, with most women experiencing a return to normal menstrual cycles within one to two months after use 2. The menstrual irregularities caused by Plan B can manifest in various ways, including changes in the timing of the next period, with some women experiencing earlier or later periods than expected, usually within a week of the anticipated date. The bleeding pattern may also differ from the normal period, potentially being lighter or heavier than usual. Some key points to consider include:
- The disruption to the menstrual cycle occurs because levonorgestrel is a synthetic progestin that temporarily alters hormone levels, affecting the timing of ovulation and the uterine lining.
- If a period is more than a week late after taking Plan B, it is advisable to take a pregnancy test, as the medication is not 100% effective at preventing pregnancy 3.
- Severe abdominal pain or unusually heavy bleeding after taking Plan B should prompt consultation with a healthcare provider, as these could indicate complications.
- The effects of Plan B on menstrual cycles are generally short-term, with most women returning to normal menstrual cycles after one to two months 4.