There is no immediate sign that tells you Plan B is working. The only reliable confirmation is getting your next period, which may come on time, a week early, or up to a week late. Until then, no symptom or side effect can confirm or rule out pregnancy. Here’s what to actually expect and when you’ll know for sure.
How Plan B Prevents Pregnancy
Plan B works primarily by delaying or preventing ovulation, the release of an egg from your ovary. If there’s no egg available, sperm can’t fertilize anything, and pregnancy doesn’t happen. It also thickens cervical mucus, which makes it harder for sperm to reach an egg in the first place.
This is why timing matters so much. If you’ve already ovulated before taking Plan B, the pill has little to no effect on preventing pregnancy. A pilot study tracking women who took emergency contraception after ovulation found that the number of pregnancies matched what researchers would have expected without any intervention at all. In other words, Plan B’s effectiveness drops dramatically once ovulation has already occurred. The problem is that most people don’t know exactly where they are in their cycle, so there’s no way to gauge in the moment how well the pill will work for you specifically.
The Only Reliable Sign It Worked
Your period is your answer. When it arrives, you can be confident that Plan B did its job. Plan B can shift your cycle by about a week in either direction, so don’t panic if your period shows up earlier or later than expected. A period that’s a few days to a week off schedule is a normal response to the hormone surge from the pill, not a sign that something went wrong.
If your period hasn’t arrived within three weeks of taking Plan B, take a pregnancy test. Home pregnancy tests are most accurate starting on the first day of a missed period. If your cycle is irregular and you’re not sure when your period is due, wait at least 21 days after the unprotected sex to test.
Side Effects Don’t Tell You Anything
Many people look for side effects as proof that Plan B is “doing something.” Nausea, spotting, fatigue, breast tenderness, and dizziness are all common after taking the pill. But these same symptoms also overlap with early pregnancy signs. Feeling nauseous the day after taking Plan B could mean the hormones are affecting your stomach, or it could mean nothing at all about whether the pill worked.
The reverse is also true. If you feel completely fine with no side effects, that doesn’t mean Plan B failed. Some people experience no noticeable changes and the pill still prevents pregnancy. There is simply no symptom, or lack of a symptom, that can serve as a reliable indicator either way.
How Timing Affects Your Odds
The sooner you take Plan B after unprotected sex, the better it works. Taken within 72 hours (three days), the pregnancy rate is about 0.8%, which translates to roughly 87 to 90% effectiveness at preventing pregnancy that would have otherwise occurred. Between 72 and 120 hours (three to five days), effectiveness drops to somewhere between 72% and 87%, with a pregnancy rate of about 1.8%.
These numbers reflect averages across all points in the menstrual cycle. Your individual odds depend heavily on where you were in your cycle when you had unprotected sex. If you were days away from ovulation, Plan B likely had enough time to delay egg release and the odds are strongly in your favor. If you were right at ovulation or just past it, the pill may not have been able to do much.
Body Weight Can Lower Effectiveness
Plan B becomes less effective at higher body weights. For women with a BMI under 25, the pregnancy rate in studies was about 1.3%. For women with a BMI between 25 and 30, it rose to 2.5%. For women with a BMI of 30 or higher, the rate jumped to 5.8%. A pooled analysis found that a BMI over 30 decreased efficacy significantly compared to lower weight categories.
If your BMI is 30 or above, a copper IUD inserted within five days of unprotected sex is the most effective form of emergency contraception regardless of weight. Another prescription emergency contraceptive pill (sold as ella) also maintains better effectiveness at higher weights than Plan B does.
Certain Medications Can Interfere
Some drugs speed up how quickly your body breaks down the hormone in Plan B, which can reduce its effectiveness. These include several anti-seizure medications (carbamazepine, phenytoin, topiramate, oxcarbazepine, felbamate), the antibiotic rifampin, the herbal supplement St. John’s wort, and certain HIV medications. If you take any of these regularly, Plan B may not work as well for you, and a copper IUD or prescription alternative would be a stronger option.
What the Waiting Period Actually Looks Like
The hardest part for most people is the uncertainty between taking the pill and getting their period. Here’s a practical timeline of what to expect. In the first one to two days after taking Plan B, you might experience nausea, headache, or fatigue. These are hormonal side effects and tell you nothing about whether pregnancy was prevented. Over the next one to three weeks, you may notice some irregular spotting or changes in your usual discharge. Again, this is a common hormonal response.
Your period should arrive within a week of its expected date. If it comes, you’re not pregnant. If it’s more than a week late, take a pregnancy test. If three weeks pass with no period, definitely test. Light spotting alone does not count as a period. You’re looking for a flow that resembles your normal menstrual bleeding.
If a pregnancy test comes back positive, Plan B did not affect the pregnancy. The pill does not harm an existing pregnancy or cause any issues for a developing embryo. It only works by preventing pregnancy from starting, not by ending one that has already begun.