Plan B is effective if you take it during your period, but the more important question is whether you actually need it. Your period is typically one of the lowest-risk times in your cycle for pregnancy, so taking Plan B at this point is unlikely to cause harm but may also be addressing a very small risk. That said, “low risk” is not “no risk,” and certain factors can change the math.
Why Pregnancy Risk During Your Period Is Low but Real
Ovulation usually happens about 13 to 14 days before your next period begins. During menstruation, you’re at the opposite end of your cycle, which is why most people assume pregnancy is impossible. For someone with a textbook 28-day cycle who ovulates mid-cycle, that assumption is largely correct.
But cycles aren’t always textbook. If your cycle is short (say, 21 to 24 days), ovulation can happen much earlier, potentially just a few days after your period ends. Sperm can survive inside the reproductive tract for up to five days after ejaculation. So if you have sex toward the end of your period and ovulate earlier than expected, viable sperm could still be present when the egg is released. This is the scenario where pregnancy from period sex actually happens.
Even people with regular cycles can have the occasional early ovulation. If you’re unsure about your cycle length or it tends to vary, the risk is harder to estimate on your own.
How Plan B Works During Menstruation
Plan B contains a synthetic hormone called levonorgestrel. Its primary job is to delay or block ovulation by suppressing the hormonal surge that triggers egg release. If ovulation doesn’t happen, there’s no egg to fertilize.
During your period, ovulation is typically still days or weeks away. Taking Plan B at this point gives the drug a wide window to suppress that hormonal surge before it even begins, which means it can be quite effective in this scenario. The closer you are to ovulation when you take it, the less reliably it works. When taken well before ovulation, as would be the case during menstruation for most people, levonorgestrel has a strong chance of preventing egg release entirely.
One important detail: Plan B does not interrupt a pregnancy that has already started. It does not affect implantation or harm a developing embryo. Its entire mechanism is about preventing ovulation.
Efficacy Rates and Timing
Plan B is 81 to 90 percent effective at preventing pregnancy, with the higher end of that range applying when you take it sooner after unprotected sex. The standard recommendation is to take it within 72 hours (three days), though it can still offer some protection up to 120 hours (five days) with reduced effectiveness.
These efficacy numbers reflect all cycle phases combined. During menstruation, when ovulation is likely still distant, Plan B’s ability to suppress that future ovulation is arguably at its strongest. The drug has more time to act before the critical window arrives.
When Plan B May Not Be the Best Option
If you have very short or irregular cycles, there’s a chance you could ovulate sooner than average. In that case, the gap between your period and ovulation shrinks, and Plan B becomes less reliable. Research comparing levonorgestrel to ulipristal acetate (sold as Ella) found a striking difference when ovulation is already close. In studies of women with large, mature follicles nearly ready to release an egg, ulipristal delayed ovulation for at least five days in about 59 percent of cycles, while levonorgestrel performed no better than a placebo at that late stage.
For most people taking Plan B during their period, this isn’t a concern because ovulation is still far off. But if your cycles are very short and you had sex near the end of a longer period, Ella may provide a wider safety margin. Ella requires a prescription, while Plan B is available over the counter.
Body Weight and Effectiveness
You may have heard that Plan B doesn’t work above a certain weight. There is no official weight limit on the FDA label. This idea originated from a 2011 analysis suggesting that women with a BMI over 30 had a significantly higher rate of pregnancy after taking levonorgestrel. However, a later investigation by the European Medicines Agency and a Cochrane review did not confirm a clear association between higher BMI and lower effectiveness, though the evidence was limited.
A small study of 10 women did find that levonorgestrel blood levels were notably lower in women with a BMI above 35 compared to those with a normal BMI. So while the data is mixed, if you’re in a higher weight range and concerned about effectiveness, talking to a pharmacist or provider about Ella or a copper IUD (which works as emergency contraception regardless of weight) is reasonable.
What to Expect After Taking It
Plan B commonly causes changes to your menstrual cycle. Your next period may come earlier or later than expected, and you might notice spotting in the days after taking the pill. Some people experience nausea, headaches, or fatigue. These effects are temporary and typically resolve within a few days.
If you take Plan B during your period, you may notice that your current period changes slightly in flow or duration, or that the following cycle is off-schedule. This is normal. If your next period is more than seven days later than expected, take a pregnancy test. For the most accurate result when you’re unsure of your cycle timing, wait at least 21 days after the unprotected sex before testing.
The Bottom Line on Period Timing
If you had unprotected sex during your period and you’re worried, Plan B will work. For most people, the drug is well-positioned to prevent ovulation because it still has days or weeks to act before an egg would be released. The real question is whether the pregnancy risk was high enough to warrant it. For someone with a regular 28-day cycle who had sex on day two, the chance of pregnancy is extremely low even without Plan B. For someone with a 23-day cycle who had sex on day five or six of their period, the risk is more meaningful, and Plan B makes more practical sense.
Either way, taking Plan B during your period won’t cause harm, won’t affect a future pregnancy, and won’t disrupt your fertility beyond possibly shifting your next cycle by a few days.