Taking Plan B while you’re on your period is safe, but it’s also the time you’re least likely to need it. Pregnancy from sex during your period is uncommon because ovulation typically happens around the middle of your cycle, not during menstruation. That said, it’s not impossible, especially if you have shorter cycles where ovulation occurs earlier. If you’ve already taken it, here’s what to expect.
Why It’s Usually Unnecessary During Your Period
Plan B works primarily by delaying or preventing ovulation. It disrupts the hormonal surge your body needs to release an egg. If that surge is blunted or shortened, the chances of a viable egg being released drop significantly. Research has shown that a short, low hormonal surge is associated with a pregnancy rate of just 5.6%, compared to 23% with a strong, two-day surge.
During your period, ovulation is still days or even weeks away for most people. Sperm can survive in the reproductive tract for up to five days, which is why pregnancy from period sex is technically possible if you ovulate early. But for someone with a typical 28-day cycle, the window doesn’t overlap. Plan B still works if you take it, but it may not have much to do since your body wasn’t close to ovulating anyway.
How It Affects Your Current Period
You may notice changes to the period you’re already having. Because Plan B delivers a large dose of a synthetic hormone, it can alter bleeding patterns even mid-period. Some people report heavier flow, while others find their period tapers off sooner than expected. In clinical studies, about 21% of participants had their cycle shortened by two or more days, and 24% had it lengthened by two or more days. Taking it in the earlier part of your cycle (which includes your period) tends to shorten the overall cycle by about a day.
Side Effects to Expect
The side effects are the same regardless of when in your cycle you take Plan B. The most common ones are headache, nausea, and cramping that feels similar to period pain. These can be harder to distinguish from normal period symptoms when you’re already menstruating, which sometimes causes confusion about whether the pill is “doing something” or your body is just doing its usual thing. Serious side effects are rare. Most symptoms resolve within a day or two.
What It Doesn’t Do
Plan B does not end a pregnancy or prevent a fertilized egg from implanting. Nine out of ten studies examining this question found no difference in uterine receptivity after taking the pill compared to controls. A review of the evidence concluded that when taken after ovulation has already occurred, it results in similar conception rates compared to placebo. Its effectiveness depends almost entirely on whether it can delay or disrupt ovulation before it happens.
Your Next Period May Shift
Even if your current period seems normal, your next one could arrive earlier or later than usual. A delay of up to one week is common. If you took Plan B during your period (the pre-ovulatory phase), your next cycle is more likely to be slightly shorter rather than longer. This is a temporary hormonal effect, not a sign of pregnancy. Your cycles should return to their normal pattern within one to two months.
If your next period is more than a week late, a pregnancy test can give you a reliable answer. The recommended timing is three weeks after taking the pill.
Weight Can Affect How Well It Works
One factor worth knowing about: body weight influences Plan B’s effectiveness at any point in your cycle. In a study of over 1,700 women, those weighing between 65 and 75 kilograms (about 143 to 165 pounds) had an estimated pregnancy rate of 1.4%. That rate jumped to roughly 6% for women weighing over 75 kilograms (165 pounds). If you weigh more than 165 pounds and want the most effective emergency contraception, a copper IUD or a different emergency contraceptive pill containing a different active ingredient may be better options to discuss with a provider.
The Bottom Line on Timing
Taking Plan B during your period won’t harm you, and it will still function the way it’s designed to. But because your fertility risk is already low during menstruation, the pill is doing less heavy lifting than it would if you took it mid-cycle. The closer you are to ovulation when you have unprotected sex, the more critical emergency contraception becomes. During your period, your body’s own timing is already working in your favor.