The most reliable sign that Plan B worked is getting your period. It may come on time, a few days early, or up to a week late, but its arrival means you’re not pregnant. Until then, there’s no immediate way to confirm the pill was effective, which understandably creates an anxious waiting period. Here’s what to expect and when you can know for sure.
How Plan B Actually Prevents Pregnancy
Plan B works by delaying ovulation, the release of an egg from your ovary. If there’s no egg available when sperm are present, fertilization can’t happen. This is the pill’s only meaningful mechanism. A review of ten studies found no difference in the uterine lining’s receptivity to a fertilized egg after taking Plan B compared to a placebo, which means the pill does not block implantation.
This matters because timing is everything. If you’ve already ovulated before taking Plan B, the pill has little to no effect. Women who took it at or after ovulation had conception rates similar to those who took a placebo. So Plan B is most effective when taken before ovulation occurs, and its effectiveness drops significantly the longer you wait after unprotected sex.
Your Period Is the Clearest Signal
Getting your period is the single best indicator that Plan B worked. It can arrive earlier than expected, right on schedule, or up to a week late. All of these are normal after taking emergency contraception. The hormonal dose in Plan B commonly shifts your cycle’s timing, so a few days’ variation in either direction isn’t a reason to worry.
You may also notice spotting or light bleeding in the days after taking the pill. This is withdrawal bleeding caused by the sudden hormone shift, not a period. It tends to be lighter and shorter than a normal period, and your PMS symptoms may feel milder than usual. This bleeding alone doesn’t confirm the pill worked, but it’s a common and expected side effect. Your actual period, arriving within its normal window (give or take a week), is what tells you pregnancy didn’t occur.
Side Effects That Mimic Early Pregnancy
One of the most stressful parts of the waiting period is that Plan B’s side effects overlap heavily with early pregnancy symptoms. Nausea, breast tenderness, fatigue, dizziness, and headaches can all show up after taking the pill. These side effects typically fade within a day or two. Early pregnancy symptoms, by contrast, tend to persist and gradually intensify over weeks.
Here’s how to tell them apart in practice:
- Nausea from Plan B usually starts within hours of taking the pill and resolves within 24 to 48 hours. Pregnancy-related nausea typically begins around week four to six and sticks around.
- Breast tenderness from Plan B is short-lived. If it lingers for more than a week and your period hasn’t arrived, that’s worth paying attention to.
- Spotting after Plan B is common and normal. But if you experience spotting instead of a full period, and it’s been more than a week past your expected date, a pregnancy test is the next step.
When to Take a Pregnancy Test
If your period is more than a week late, take a pregnancy test. Planned Parenthood recommends testing if your period hasn’t arrived within three weeks of taking Plan B. A home pregnancy test won’t give an accurate result until at least ten days after conception, because the hormone it detects needs time to build up. Waiting a full two weeks after the unprotected sex reduces the chance of a false negative.
If you test too early and get a negative result but your period still hasn’t come, test again a few days later. A single negative test with a missing period isn’t definitive. Two negative tests spaced a few days apart, taken at least two weeks after the event, give you a much more reliable answer.
Factors That Reduce Effectiveness
Plan B is not equally effective for everyone. Body weight plays a significant role. Research from Oregon Health and Science University found that people with a BMI of 30 or higher experienced Plan B failure four times as often as those with a BMI under 25. The reason is pharmacological: blood levels of the active drug were 50% lower in individuals with a higher BMI after a standard dose, meaning the drug likely never reached the concentration needed to suppress ovulation. Doubling the dose did not solve this problem.
Timing also matters enormously. Plan B is most effective within the first 24 hours and becomes progressively less reliable over the next 48 to 72 hours. And if you’d already ovulated before taking it, the pill offers essentially no additional protection. There’s no way to know exactly when you ovulate without tracking, which is part of why effectiveness rates vary so widely from person to person.
What About Taking It More Than Once?
If you had unprotected sex on separate occasions in the same cycle, you can take Plan B each time. There is no limit on how many times you can use it, and repeated use of levonorgestrel-based pills is not associated with long-term side effects. However, taking an extra dose after the same single encounter won’t make it more effective.
One important exception: if you’ve taken ella (a different type of emergency contraception) since your last period, do not take Plan B. The two drugs can counteract each other and both become ineffective. They should not be taken together or within five days of each other.
Symptoms That Need Immediate Attention
In rare cases, pregnancy occurs despite emergency contraception, and occasionally that pregnancy can be ectopic, meaning the fertilized egg implants outside the uterus, usually in a fallopian tube. This is a medical emergency. Warning signs include severe abdominal or pelvic pain paired with vaginal bleeding, extreme lightheadedness or fainting, and shoulder pain. If you experience any combination of these, seek emergency care immediately. These symptoms can appear even before you’d expect a missed period.
Outside of that rare scenario, the waiting game after Plan B is mostly about patience. Your period is the answer. If it comes, you’re not pregnant. If it doesn’t come within three weeks, a pregnancy test will give you clarity.