What Is Levonorgestrel?
Levonorgestrel is a synthetic progestin, a man-made version of the hormone progesterone, which is essential for regulating the menstrual cycle and pregnancy. In emergency contraception (EC) pills like Plan B One-Step, levonorgestrel is taken as a high, single dose after unprotected sex. It is also used in lower doses in regular birth control pills and intrauterine devices (IUDs).
The primary purpose of the high-dose levonorgestrel in EC is to prevent pregnancy by interfering with ovulation, the release of an egg from the ovary. By delaying or stopping the release of the egg, it prevents fertilization. This is the central mechanism through which it can alter the timing and characteristics of your next period.
The Mechanism Behind Menstrual Changes
The menstrual cycle is a finely tuned process controlled by hormonal fluctuations, particularly those of estrogen and progesterone. The high dose of levonorgestrel floods the body with a synthetic hormone, causing a temporary disruption to this delicate balance. The precise effect on your period depends heavily on where you are in your cycle when you take the pill.
Effect of Timing on Your Period
- Early in the cycle (before ovulation): If you take levonorgestrel before the luteinizing hormone (LH) surge that triggers ovulation, the medication can block or delay that surge. This can cause your period to arrive earlier than expected, effectively shortening your cycle. The subsequent cycle should return to its normal rhythm.
- Near or at ovulation: Taking the pill very close to or during ovulation has a reduced effect on preventing ovulation and may not significantly change the cycle length. However, the hormonal disruption can still lead to a delayed or otherwise irregular period.
- After ovulation: If ovulation has already occurred, levonorgestrel is much less effective at preventing pregnancy, though some speculate about potential effects on the uterine lining. Nonetheless, it can still cause a delayed period or other bleeding changes. Research has found no evidence that levonorgestrel EC has a post-ovulatory effect on implantation.
Other Common Side Effects
Besides changes to your period, it is common to experience other side effects after taking levonorgestrel. These are typically mild and resolve quickly as your hormone levels return to normal.
Potential side effects from the morning-after pill include:
- Nausea and vomiting
- Headaches and dizziness
- Fatigue
- Breast tenderness
- Lower abdominal cramps or pain
- Spotting or bleeding between periods
If vomiting occurs within two hours of taking the pill, it may need to be repeated, so you should contact a healthcare professional for guidance. For most people, side effects are not severe and last only a day or two.
Differentiating Spotting from Your Period
One of the most confusing side effects is unscheduled bleeding. It is important to distinguish this from your next period.
Key differences between spotting and a period after taking levonorgestrel:
- Timing: Spotting typically occurs a few days after taking the pill, as a direct result of the hormonal fluctuations. Your actual period will arrive closer to its expected time, though it may be delayed or early.
- Flow: Spotting is much lighter than a regular period and usually lasts only a day or two. A period will have a typical menstrual flow, although the heaviness may be different than usual.
- Duration: Spotting is short-lived, while a period will follow its normal duration (around 3-7 days).
If you experience bleeding that is unusually heavy, prolonged (longer than 7 days), or accompanied by severe abdominal pain, you should contact a healthcare provider to rule out other issues.
When to Contact a Healthcare Provider
While most changes are temporary and not a cause for concern, some symptoms warrant medical attention. You should reach out to a healthcare provider if:
- Your period is more than a week late.
- You have severe abdominal pain or cramping, as this could indicate an ectopic pregnancy.
- You experience excessive bleeding, such as soaking through two or more pads or tampons in an hour for a couple of hours.
- Your bleeding lasts for an unusually long duration.
- You have any signs of a potential allergic reaction, such as hives, swelling, or difficulty breathing.
Comparison of Emergency Contraceptives
Besides levonorgestrel, another common type of emergency contraception is ulipristal acetate (Ella). Here is a comparison of some key characteristics for understanding how they affect your cycle based on information from the National Institutes of Health (NIH):
| Feature | Levonorgestrel (e.g., Plan B) | Ulipristal Acetate (e.g., Ella) |
|---|---|---|
| Effect on Period | Can cause an early, late, heavier, or lighter period. The effect is highly dependent on cycle timing. | Can also cause period changes, but may be more likely to cause a delayed period. |
| Effective Window | Most effective when taken within 72 hours of unprotected sex, but can be used for up to 120 hours (5 days). | Works for up to 120 hours (5 days) after unprotected sex. |
| Mechanism | Primarily works by delaying or preventing ovulation. | Also works by delaying or preventing ovulation, and may delay it later than levonorgestrel. |
| Weight Impact | May be less effective for those with a higher body mass index (BMI). | Seems to maintain its effectiveness across a broader range of BMI. |
| Side Effects | Similar range of side effects, including nausea, headache, dizziness, and breast tenderness. | Similar to levonorgestrel, with occasional mild side effects. |
Conclusion
In summary, levonorgestrel can and often does trigger changes to your menstrual cycle, but it does so by disrupting the hormonal process rather than inducing a period on demand. The effect is not uniform and can result in a period that is earlier, later, lighter, or heavier than usual, depending on when in the cycle the pill is taken. Spotting or unscheduled bleeding is also a common side effect. These changes are temporary, with the cycle typically returning to its normal pattern within a month or two. While these alterations can be confusing, knowing what to expect is key. If your period is more than a week late, a pregnancy test is recommended to confirm the pill's effectiveness. Always consult a healthcare provider with any persistent concerns or severe symptoms. For additional guidance, consider the information provided by the Cleveland Clinic regarding emergency contraception.
