The single most reliable sign that Plan B worked is getting your next period. Once your period arrives, even if it’s earlier or later than expected, that confirms you’re not pregnant. If your period doesn’t come within three weeks of taking the pill, take a pregnancy test.
That waiting period can feel stressful, especially because Plan B itself causes side effects that overlap with early pregnancy symptoms. Here’s how to make sense of what your body is doing in the days and weeks after taking it.
Your Period Is the Clearest Answer
Plan B commonly shifts your cycle. Your period may arrive a few days early, a week late, or right on time. The flow might be heavier, lighter, or spottier than usual. All of these variations are normal side effects of the pill’s high dose of hormones, and none of them tell you whether it worked or failed. What matters is that bleeding resembling a period shows up at all.
Some people experience spotting within a few days of taking Plan B. This is withdrawal bleeding triggered by the pill, not a period, and it doesn’t confirm anything either way. You still need to wait for your actual period to arrive. If three weeks pass with no period, a home pregnancy test will give you an accurate result at that point.
Side Effects That Mimic Pregnancy
Plan B can cause nausea, fatigue, dizziness, breast tenderness, and headaches. These are temporary hormonal side effects that typically fade within a day or two. The problem is that early pregnancy causes nearly identical symptoms: nausea, fatigue, dizziness, breast changes, and increased urination. There is no way to tell the difference based on how you feel alone.
This is why symptoms are not a reliable indicator in either direction. Feeling fine doesn’t mean Plan B worked, and feeling nauseous a week later doesn’t mean you’re pregnant. The only tools that give a real answer are your period and, if it’s late, a pregnancy test.
Spotting vs. Implantation Bleeding
Light spotting after Plan B is common and expected. But implantation bleeding, which happens when a fertilized egg attaches to the uterine lining, also looks like light spotting. This can create confusion.
Implantation bleeding typically occurs 10 to 14 days after ovulation. It’s usually pink or brown (not bright red), very light, lasts one to two days, and doesn’t soak through a pad. If your spotting is heavier, contains clots, or is bright red, it’s more likely a Plan B side effect or an early period than implantation bleeding. Still, timing and color alone aren’t definitive. A pregnancy test after three weeks without a period remains the most reliable next step.
How Plan B Actually Works
Understanding what Plan B does helps explain why it sometimes fails. The pill works by delaying ovulation, the release of an egg from the ovary. If ovulation hasn’t happened yet when you take it, the pill can prevent the egg from being released, meaning sperm have nothing to fertilize.
If ovulation has already occurred before you take the pill, Plan B is significantly less effective. Research published in Contraception found that when levonorgestrel (the active ingredient) is taken after ovulation, conception rates are similar to those expected after unprotected sex with no emergency contraception at all. The pill does not prevent a fertilized egg from implanting. This means the timing of your cycle when you had unprotected sex matters enormously, but most people don’t know exactly when they ovulate, which makes it impossible to predict effectiveness in advance.
Timing and Weight Affect Effectiveness
Plan B is most effective the sooner you take it. Within the first 24 hours, it prevents about 95% of expected pregnancies. The overall effectiveness across the full 72-hour window drops to about 89%. After 72 hours, it’s generally not recommended because the benefit declines sharply.
Body weight also plays a role. If you weigh more than 165 pounds, Plan B may not work as well. The hormones in the pill are less effective at higher body weights, and there’s no option to simply double the dose. If this applies to you and your period is late, taking a pregnancy test sooner rather than later is a reasonable approach.
One Important Rule About Vomiting
If you vomit within two hours of swallowing the pill, your body may not have absorbed enough of the medication. In that case, you need to take another dose. If you kept the pill down for more than two hours, vomiting afterward won’t reduce its effectiveness.
What a Missed Period Means
A missed period is the most significant sign that Plan B may not have worked. But “missed” needs context: Plan B can delay your period by several days or even a week, which doesn’t automatically mean pregnancy. The three-week mark is the threshold recommended by Planned Parenthood for taking a pregnancy test.
A positive test at that point is definitive. A negative test is also reliable if taken three weeks or more after you took the pill. If your test is negative but your period still hasn’t come after another week, testing again or following up with a healthcare provider can help rule out other causes for the delay.