The most reliable way to know if Plan B worked is to wait for your next period. If it arrives on time, or even a little early or late, the pill most likely did its job. If your period is more than seven days late, take a pregnancy test. There’s no immediate sign right after swallowing the pill that tells you it worked or didn’t.
That waiting period can feel stressful, so here’s what to expect in the days and weeks ahead, what’s normal, and when you actually need to worry.
How Plan B Prevents Pregnancy
Plan B contains a hormone that stops or delays your ovary from releasing an egg. If there’s no egg available when sperm arrive, fertilization can’t happen. That’s the entire mechanism. It does not end an existing pregnancy, and the FDA confirms it will not affect a pregnancy that has already started. If you were already pregnant before you took it, Plan B simply won’t do anything.
This is also why timing matters so much. The pill is most effective when taken as soon as possible after unprotected sex, ideally within 24 hours. It can still work up to 72 hours afterward, but effectiveness drops with every passing day. By the time you’re past the three-day window, it’s significantly less reliable.
What Your Period Will Tell You
Your period is the single best indicator. After taking Plan B, it’s completely normal for your next cycle to look different from usual. It might come a few days early or a few days late. The flow could be heavier, lighter, or spottier than you’re used to. None of these changes mean the pill failed.
The key threshold to watch: if your period is more than seven days later than expected, take a pregnancy test. Planned Parenthood recommends testing three weeks after taking the pill if your period hasn’t shown up by then. A standard home pregnancy test from a drugstore is accurate at that point.
Spotting and Bleeding Between Periods
Some people notice light bleeding or spotting a few days after taking Plan B, well before their period is due. This is called breakthrough bleeding, and it’s a common side effect of the hormone surge, not a period and not a sign of pregnancy. It can range from a few spots on your underwear to light bleeding that lasts a day or two. It doesn’t confirm the pill worked, but it’s not a red flag either.
Side Effects That Mimic Early Pregnancy
This is where things get confusing. Plan B’s side effects overlap almost entirely with early pregnancy symptoms, which makes it impossible to tell the difference based on how you feel alone. Common side effects of the pill include:
- Nausea or vomiting
- Breast tenderness
- Fatigue
- Headaches and dizziness
- Cramping or abdominal pain
Every one of those is also a symptom of early pregnancy. So if you’re feeling nauseous or tired a few days after taking Plan B, that’s almost certainly the pill itself. These side effects typically fade within a day or two. If they persist for weeks, or if you develop new symptoms after the initial side effects cleared up, that’s worth paying attention to, but your period (or a pregnancy test) will still give you the most definitive answer.
When a Pregnancy Test Becomes Reliable
Home pregnancy tests detect a hormone that your body only produces after a fertilized egg implants in the uterus. That process takes time. Testing too early, like a few days after taking Plan B, will almost always give you a negative result regardless of whether you’re pregnant. The test simply can’t detect anything yet.
The reliable window is three weeks after taking Plan B. At that point, if you are pregnant, your hormone levels will be high enough for a standard home test to pick up. If the test is negative at the three-week mark, you can trust that result. If your period still hasn’t arrived but the test is negative, wait a few more days and test again. Plan B can shift your cycle enough that your period is just running behind schedule.
Factors That Affect Whether Plan B Works
Timing is the biggest factor. The sooner you take it after unprotected sex, the better your odds. But it’s not the only variable.
Body weight plays a role. Research has found that Plan B is less effective in people who weigh more than 165 pounds. Some studies showed up to double the risk of pregnancy in people with higher body weights compared to smaller individuals. If this applies to you, a copper IUD inserted within five days of unprotected sex is a more reliable emergency contraceptive option, and your provider can discuss alternatives that may work better at higher weights.
Where you are in your cycle also matters. If you’ve already ovulated before taking the pill, Plan B can’t do what it’s designed to do, because the egg has already been released. There’s no way to know for sure whether ovulation had already happened, which is part of why the pill isn’t 100% effective even when taken immediately.
Signs the Pill May Not Have Worked
The clearest sign is a missed period, specifically one that’s more than a week late. Beyond that, watch for symptoms that develop in the weeks after taking Plan B rather than in the first day or two. Persistent nausea, increasing breast tenderness, or fatigue that doesn’t go away could point to early pregnancy rather than lingering side effects.
If you experience severe abdominal pain or heavy vaginal bleeding (not spotting, but soaking through pads), contact a healthcare provider. These can be signs of an ectopic pregnancy or miscarriage, both of which need medical attention regardless of whether you took Plan B.
For most people, the answer is straightforward: take the pill, wait for your period, and test at three weeks if it doesn’t come. The waiting is the hardest part, but your body will give you a clear answer.