Taking Plan B while you’re on your period is safe, and it works the same way it would at any other point in your cycle. Your risk of pregnancy from sex during your period is lower than at other times, but it’s not zero, so taking emergency contraception can still be a reasonable precaution. The main thing you’ll likely notice is some changes to your current or next period.
Why You Can Get Pregnant During Your Period
The reason Plan B isn’t pointless during your period comes down to two biological realities. First, ovulation doesn’t always happen on schedule. If you have a short cycle (under 24 days), you may ovulate much earlier than expected, sometimes while you’re still bleeding. Second, sperm can survive inside the body for up to five days after ejaculation. So even if you aren’t ovulating during your period, sperm from sex on day 4 of your cycle could still be viable on day 9, when an egg might be released.
This combination of early ovulation and sperm longevity is exactly why doctors don’t consider menstruation a reliable form of birth control.
How Plan B Works in Your Cycle
Plan B contains a synthetic hormone called levonorgestrel, and its primary job is to delay or prevent ovulation. If your body hasn’t released an egg yet, the pill keeps that from happening, so sperm have nothing to fertilize.
This is important context for taking it during your period. Early in your cycle, ovulation is likely still days away. Plan B is most effective when taken before the hormonal surge that triggers egg release. If you’re menstruating, that surge almost certainly hasn’t happened yet, which means the pill is working in its ideal window. A review published in the journal Contraception confirmed that when levonorgestrel is taken before this hormonal surge, it effectively blocks ovulation. After ovulation has already occurred, the pill does not prevent implantation and results in conception rates similar to placebo.
In short, taking Plan B during your period may actually be one of the more effective times to use it, because your body is still in the early pre-ovulation phase.
Effects on Your Current Period
Plan B delivers a concentrated dose of hormones, and your body will respond to that even if you’re already bleeding. The most common changes people notice fall into a few categories:
- Heavier or lighter flow. Some people see their current period become heavier after taking the pill, while others notice it lightens. Neither is cause for concern.
- Longer duration. Research shows that taking emergency contraception at various points in the menstrual cycle, including early on, can make a period last longer than usual.
- Spotting between periods. Breakthrough bleeding or spotting in the days or weeks after taking Plan B is common and isn’t a sign that anything is wrong.
These changes are more likely if you’ve taken Plan B more than once in the same menstrual cycle.
Effects on Your Next Period
Even if your current period seems normal after taking Plan B, your next one may shift. This is one of the most commonly reported side effects of emergency contraception. Your next period could come earlier or later than expected, with the delay potentially reaching up to one week. It may also be heavier, lighter, or more spotty than your usual cycle.
This happens because the burst of synthetic hormones temporarily disrupts your cycle’s timing. Your body essentially needs to recalibrate. For most people, the cycle after that returns to normal.
Other Side Effects to Expect
Because you’re already dealing with period symptoms like cramping and bloating, it can be hard to separate what’s your period and what’s the pill. Common Plan B side effects include nausea, fatigue, headache, and lower abdominal pain. If you’re already experiencing cramps during your period, you may notice them intensifying slightly. Nausea is the most distinctive side effect that you can clearly attribute to the pill rather than your cycle. It typically passes within a day or two.
Weight and Effectiveness
One factor worth knowing about regardless of when you take Plan B: its effectiveness decreases as body weight increases. The Society of Family Planning has acknowledged this relationship in its clinical guidance. If this is a concern for you, a copper IUD inserted as emergency contraception is the most effective option at any weight and can be placed up to five days after unprotected sex.
When to Take a Pregnancy Test
If you want confirmation that Plan B worked, take a home pregnancy test three weeks after the day you took the pill. Testing earlier than that can give inaccurate results because your body may not have produced enough pregnancy hormone to detect. Don’t rely on your next period arriving as proof that you’re not pregnant, especially since Plan B itself can delay or alter that period. The three-week mark gives you a reliable answer.