There is no immediate way to know if Plan B worked. The only reliable confirmation is getting your period, which may arrive on time, early, or up to a week late. If your period doesn’t come within three weeks of taking Plan B, take a pregnancy test.
That waiting period can feel stressful, especially when your body is throwing confusing signals. Side effects from the pill itself can mimic early pregnancy symptoms, making it hard to read what’s happening. Here’s what to actually watch for and what the timeline looks like.
Side Effects Are Not Signs It Worked
Plan B delivers a large dose of synthetic hormone, and your body will likely react. In clinical trials, the most common side effects were heavier menstrual bleeding (31% of women), nausea (14%), lower abdominal pain (13%), fatigue (13%), and headache (10%). Breast tenderness, dizziness, and vomiting also showed up in a meaningful number of users.
These side effects overlap almost entirely with early pregnancy symptoms, which is why you can’t use how you feel as evidence that the pill worked or didn’t. Nausea after taking Plan B is a reaction to the hormone surge, not a sign of anything else. Feeling fine afterward doesn’t mean it failed, and feeling awful doesn’t mean it succeeded. Your body’s reaction to the drug has no connection to whether it prevented pregnancy.
Your Period Is the Real Answer
The clearest sign Plan B worked is getting your period. It may not look exactly like your normal period. More than half of women in clinical studies got their period within two days of when they expected it, but 31% experienced some change in their bleeding pattern. About 4.5% had their period arrive more than seven days late.
Your period might come a few days early, arrive right on schedule, or show up late. It could be heavier or lighter than usual, or more spotty. All of this is normal after taking emergency contraception. The key threshold: if three weeks pass after taking Plan B and you still haven’t had any period, take a pregnancy test.
Spotting vs. Implantation Bleeding
Some women notice light spotting a few days after taking Plan B, which can trigger worry about implantation bleeding. These two things look different if you know what to check. Plan B spotting typically shows up within a few days of taking the pill, can range from light to moderate, and lasts two to three days on average (sometimes up to a week). About 15% of women in one study experienced this kind of bleeding roughly four days after taking the pill.
Implantation bleeding, by contrast, happens 10 to 14 days after ovulation. It’s very light, often just faint spotting, and lasts only a few hours to two days. Timing is the most useful distinction. If you see spotting two to four days after taking Plan B, that’s almost certainly a side effect of the drug, not implantation.
When to Take a Pregnancy Test
A pregnancy test needs enough time to detect the hormone produced during pregnancy. Taking one too early will give you an unreliable result. The standard recommendation from Planned Parenthood is to wait until three weeks after you took Plan B. At that point, a standard home pregnancy test is accurate enough to trust.
If your period arrives before that three-week mark, even if it looks a bit different from normal, you can generally take that as confirmation you’re not pregnant. A full menstrual bleed (not just light spotting) means the uterine lining is shedding, which is what happens when pregnancy hasn’t occurred.
Factors That Affect Whether Plan B Worked
Plan B is estimated to be 60% to 94% effective when taken within 72 hours of unprotected sex. That’s a wide range because effectiveness drops significantly with every passing hour. Taking it within the first 24 hours gives you the best odds. By the third day, it’s considerably less reliable.
The pill works primarily by delaying or preventing ovulation. If your body has already released an egg, Plan B is much less likely to help. This is why timing matters so much, and it’s also why the pill doesn’t work every time: if you happened to take it after ovulation had already occurred, there’s a real chance it won’t prevent pregnancy.
Body weight also plays a role. Research has found that Plan B becomes less effective for women with a BMI above 26. The specific type of hormone in Plan B is sensitive to weight, and for women at higher BMIs, a copper IUD inserted within five days of unprotected sex is a more reliable emergency contraception option.
Certain medications can also reduce Plan B’s effectiveness. Anti-seizure medications like phenytoin, carbamazepine, and topiramate, along with the antibiotic rifampin (used for tuberculosis), can interfere with how your body processes the hormone. If you take any of these regularly, Plan B may not work as well for you.
Going Back to Regular Birth Control
You don’t need to wait for your next period to restart hormonal birth control after taking Plan B. You can begin or resume pills, the patch, or the ring immediately. If you were already on the pill and needed Plan B because of a missed dose, just continue your current pack as normal.
One exception: if you took ella (a different emergency contraceptive that uses a different active ingredient), you should wait six days before starting or restarting hormonal birth control. Starting sooner can reduce the effectiveness of both the emergency contraceptive and your regular method. During that waiting period and for seven days after restarting your hormonal method, use condoms as backup.