From the Guidelines
Plan B can affect your normal menstrual cycle, causing changes such as earlier or later periods, heavier or lighter bleeding, and irregularity, as it works by delaying or preventing ovulation through a high dose of levonorgestrel. These changes are temporary and usually resolve by your next menstrual cycle 1. After taking Plan B (levonorgestrel 1.5 mg), you might experience:
- Changes to your next period, which could come earlier or later than expected, typically within a week of your normal time
- Heavier, lighter, or more irregular bleeding than usual
- Spotting or light bleeding shortly after taking the medication These effects occur because Plan B disrupts your body's normal hormonal patterns, and if your period is more than a week late after taking Plan B, you should take a pregnancy test to rule out pregnancy 1. While these cycle changes can be concerning, they don't indicate any long-term problems, and your cycle should return to normal within 1-2 months. It's essential to note that the recommended dosage of levonorgestrel is a single 1.5-mg dose, available either as 2 pills (0.75 mg each) or as 1 pill (Plan B One-Step), and it's crucial to follow the prescribed dosage to minimize potential side effects 1. If you experience severe abdominal pain or your period remains irregular for several months after taking Plan B, consult a healthcare provider.
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. 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. If your period is delayed beyond 1 week, it is possible you may be pregnant
Levonorgestrel tablet may affect your normal cycle, causing changes in your period, such as a period that is heavier or lighter, or a period that is early or late 2.
- Some women may experience menstrual changes, including tiredness, dizziness, nausea, headache, breast pain, and lower stomach pain.
- Your next period should come at the expected time or within a week of the expected time after taking levonorgestrel tablet.
- If your period is more than a week late, you may be pregnant and should get a pregnancy test and follow up with your healthcare professional 2.
From the Research
Effects of Plan B on Menstrual Cycle
- The administration of levonorgestrel, the active ingredient in Plan B, can affect the menstrual cycle in several ways, including delaying the luteinizing hormone (LH) surge and interfering with ovulation 3, 4.
- Studies have shown that taking Plan B before ovulation can delay the onset of the next menses, while taking it after ovulation may not have a significant effect on the cycle length 3, 5.
- The effectiveness of Plan B in preventing pregnancy is higher when taken before ovulation, with one study showing a significant reduction in expected pregnancies when taken before ovulation 6, 7.
- The use of Plan B can also disrupt corpus luteum formation, causing premature luteinization of unruptured follicles, which can affect the menstrual cycle 3.
Bleeding Patterns and Pituitary-Ovarian Function
- The administration of Plan B can affect bleeding patterns, with some studies showing a shortening of the cycle length when taken in the follicular phase 5.
- The use of Plan B can also suppress ovulation when taken in the late preovulatory phase, prior to the gonadotrophin surge 5.
- The effects of Plan B on pituitary-ovarian function can vary depending on the timing of administration, with some studies showing a significant impact on LH and progesterone levels 3, 4.