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 your period shows up (or doesn’t), there’s no way to know for certain whether emergency contraception was successful. No specific symptom or side effect confirms it worked, and no early symptom reliably confirms it failed. The only definitive answer, if your period is late, comes from a pregnancy test.
What Your Next Period Tells You
Plan B’s main job is to delay or prevent ovulation, which means it disrupts your normal cycle. Most women get their period within two days to one week of when they expected it. Some get it a few days early. Both of these outcomes are normal and are the clearest signal that the pill did its job.
If your period is more than a week late, that’s the point to take a pregnancy test. A delay of a few days on its own isn’t a red flag since Plan B commonly shifts your cycle. But a period that never arrives is the single strongest indicator that the pill may not have worked.
Spotting After Plan B Is Normal
Many women notice light bleeding or spotting within the first few days after taking Plan B. This is sometimes called withdrawal bleeding, and it’s a side effect of the hormone dose, not a period. It typically lasts two to three days and ranges from light to moderate. This spotting doesn’t confirm or deny pregnancy. It’s simply your body reacting to a large surge of synthetic hormone.
Implantation bleeding, by contrast, happens 10 to 14 days after ovulation and looks different. It’s usually very light (a few hours to two days of faint spotting) and occurs later in your cycle. If you notice light spotting that starts a week or more after taking Plan B, pay attention to the timing. Spotting right after taking the pill is almost certainly a drug side effect. Spotting that appears much later could, in rare cases, be implantation, though plenty of other things cause mid-cycle spotting too.
Side Effects That Don’t Mean It Failed
Plan B side effects and early pregnancy symptoms overlap in frustrating ways. Nausea, fatigue, breast tenderness, dizziness, and headaches can all show up after taking emergency contraception. These same symptoms are also associated with very early pregnancy. There is no way to tell the difference based on how you feel.
If your symptoms appear within the first day or two after taking the pill, they’re most likely side effects. Plan B contains a high dose of levonorgestrel (the same hormone found in many birth control pills), and that dose commonly causes short-term nausea and fatigue. These side effects usually fade within 24 to 48 hours. If nausea, fatigue, or breast tenderness persist or worsen over the following weeks, a pregnancy test is the only reliable next step.
When Plan B Is Less Likely to Work
Understanding the situations where Plan B is less effective can help you gauge your own risk.
- Timing relative to ovulation. Plan B works primarily by delaying ovulation. If you had already ovulated before taking it, the pill has little to no effect. Research has shown that when levonorgestrel is given on the day of ovulation or after, pregnancy rates are essentially the same as if no emergency contraception was taken. This is why taking it as soon as possible matters: the earlier you take it, the more likely it is to block ovulation before it happens.
- Body weight. Plan B may be less effective in women who weigh more than 165 pounds and may offer minimal protection above 176 pounds. Women with a BMI over 25 may want to discuss alternatives with a pharmacist or provider, such as the copper IUD or a different emergency contraceptive pill designed for higher body weights. That said, levonorgestrel still provides some benefit at higher weights, so taking it is better than skipping it if no alternative is available.
- Vomiting after the dose. If you vomit within about 15 minutes of swallowing the pill, or if you can see the intact tablet in your vomit, you likely need to take another dose. After roughly an hour, the medication has generally moved past your stomach and vomiting won’t affect absorption.
When and How to Take a Pregnancy Test
Home pregnancy tests detect a hormone that your body only produces after a fertilized egg implants in the uterus. That process takes time, so testing too early gives unreliable results. The NHS recommends waiting at least 21 days after unprotected sex if you’re unsure when your period is due. If you do have a predictable cycle, you can test from the first day of a missed period.
Testing before three weeks have passed risks a false negative, meaning the test says you’re not pregnant when you actually are. If you get a negative result but your period still hasn’t arrived a few days later, test again. A positive result at any point is reliable and means Plan B did not prevent pregnancy.
The Realistic Success Rate
Plan B reduces the risk of pregnancy from a single act of unprotected sex by about 85% when taken within 72 hours. That number drops the longer you wait. It does not guarantee prevention even under ideal circumstances. If you took the pill quickly, hadn’t yet ovulated, and your period arrives within a week of its expected date, you can be very confident it worked. If your period is significantly late, a pregnancy test is the only way to get a clear answer. No combination of symptoms, spotting patterns, or gut feelings can substitute for that test.