The most reliable sign that Plan B worked is getting your period. It may come at its usual time, a few days early, or up to a week late, but its arrival means you’re not pregnant. If your period doesn’t show up within three weeks of taking the pill, a pregnancy test will give you a definitive answer.
That waiting period can feel long, especially when Plan B’s own side effects mimic early pregnancy. Here’s what to expect and how to tell the difference.
How Plan B Actually Prevents Pregnancy
Plan B works by stopping or delaying ovulation, the release of an egg from your ovary. If there’s no egg available, sperm can’t fertilize anything. The FDA has confirmed that the drug acts on ovulation and has no direct effect on fertilization or implantation. It also won’t end an existing pregnancy.
This is important context for understanding whether it worked for you. If you had already ovulated before taking the pill, Plan B is significantly less effective because its primary mechanism is no longer in play. You have no way to know exactly when you ovulated, which is part of why the waiting game exists.
Your Period Is the Main Signal
Getting your period is the clearest confirmation that Plan B did its job. Most people get their next period within a week of when they’d normally expect it. Some get it a few days early. A delay of up to one week is common and not a reason to worry on its own.
That said, your period might look different than usual. It’s common for the first post-Plan B period to be heavier, lighter, or more spotty than normal. The timing, flow, and duration can all shift. These changes are caused by the high dose of hormone in the pill and don’t signal a problem.
You might also notice spotting days or even a week or more before your actual period arrives. This breakthrough bleeding is a side effect of the medication, not a true period. It doesn’t confirm or rule out pregnancy on its own, but it’s a normal response to the drug.
When to Take a Pregnancy Test
If your period hasn’t arrived within three weeks of taking Plan B, take a home pregnancy test. At that point, a test is reliable enough to give you a clear answer. Testing too early can produce a false negative because your body may not have built up enough pregnancy hormone to detect.
If you know when your period was due, testing from the first day of that missed period is also a reliable approach. For the most accurate result, use your first urine of the morning, when hormone concentration is highest. A negative test at the three-week mark is a strong indicator that Plan B worked.
Side Effects That Look Like Pregnancy
Plan B can cause nausea, breast tenderness, headaches, dizziness, and fatigue. These overlap almost perfectly with early pregnancy symptoms, which makes the waiting period even more stressful. The key difference is timing: Plan B side effects typically show up within hours to a couple of days after taking the pill and fade quickly. Early pregnancy symptoms don’t usually appear until at least two to three weeks after conception.
If you feel nauseated or have sore breasts in the first few days after taking Plan B, that’s far more likely a reaction to the medication than a sign of pregnancy. These symptoms on their own tell you nothing about whether the pill worked.
How Effective Plan B Is by Timing
Plan B’s effectiveness drops sharply with every day you wait. Taken within the first 24 hours after unprotected sex, it’s about 94% effective at preventing pregnancy. By 72 hours, that number falls to roughly 58%. This is why speed matters so much, and it’s also why the pill doesn’t work 100% of the time even under ideal conditions.
Your body weight also plays a role. Research cited by the American College of Obstetricians and Gynecologists indicates that Plan B may be less effective in people with a BMI of 25 or higher, with efficacy declining further at higher weights. If this applies to you, a copper IUD inserted within five days of unprotected sex is the most effective emergency contraception option regardless of weight. Another prescription emergency contraceptive pill may also retain effectiveness at higher weight thresholds than Plan B does.
Signs That Plan B May Not Have Worked
A period that’s more than a week late is the first real reason to suspect the pill didn’t work. Other early signs of pregnancy to watch for include persistent nausea (especially if it starts two or more weeks after you took the pill), unusual fatigue, frequent urination, and breast changes that don’t resolve.
In rare cases where emergency contraception fails, there’s a small increased consideration for ectopic pregnancy, where a fertilized egg implants outside the uterus. Sharp or persistent pelvic pain combined with a missed period or unusual bleeding warrants prompt medical attention, as ectopic pregnancy can become a serious emergency.
The Short Version of What to Watch
- Period arrives on time or within a week of its expected date: Plan B almost certainly worked.
- Spotting before your period: Normal side effect. Not a period, but not a danger sign.
- Nausea or breast tenderness in the first few days: Side effect of the medication, not pregnancy.
- Period is more than a week late: Take a pregnancy test.
- No period within three weeks: Take a pregnancy test regardless of other symptoms.