After taking Plan B, most people feel completely normal or experience only mild side effects like nausea or a headache that resolve within a day or two. The pill works by delaying ovulation, and your body processes it quickly. The bigger changes show up later, when your next period arrives earlier or later than expected.
How Plan B Works in Your Body
Plan B contains a synthetic hormone that delays or prevents your ovary from releasing an egg. If there’s no egg available, sperm have nothing to fertilize. This is the pill’s primary mechanism, and it’s why timing matters so much: the sooner you take it after unprotected sex, the more likely it is to intervene before ovulation occurs.
Plan B does not end an existing pregnancy. If a fertilized egg has already implanted, the pill has no effect. It also becomes less effective the longer you wait. Taking it within the first 24 hours gives you the best chance of preventing pregnancy, and effectiveness continues to drop through the 72-hour window.
Side Effects in the First Few Days
The most common side effects are mild and short-lived. You may experience nausea, a headache, dizziness, lightheadedness, or breast tenderness. These typically fade within a day or two as your body processes the dose of hormone.
Some people also notice spotting, which is light bleeding that shows up as a few spots of pink, red, or brown blood on toilet paper or underwear. This is not a period. It’s a normal hormonal response and can start within a few days of taking the pill. If you’re soaking through a pad every hour for two hours or more, or you develop severe abdominal pain, those are signs that need medical attention. Severe abdominal pain in particular can indicate an ectopic pregnancy, which is a medical emergency.
What Happens to Your Next Period
This is the part that catches most people off guard. Plan B commonly shifts the timing of your next period by up to a week in either direction. Your period arriving a week early or a week late is completely normal after taking the pill. You may also notice that your flow is heavier or lighter than usual.
If your period is more than a week late, it’s worth taking a pregnancy test. The most reliable time to test is the first day of a missed period, or at least 21 days after unprotected sex if you’re unsure when your period is due. A test taken too early can give a false negative because pregnancy hormone levels haven’t built up enough to detect.
Factors That Reduce Effectiveness
Body weight plays a role that many people don’t realize. Research has found that Plan B becomes less effective for people with a BMI above 26. If your BMI falls in that range, a different type of emergency contraception (the prescription option sold as ella) or a copper IUD may be more reliable choices.
Certain medications also interfere with how your body absorbs Plan B. These include the antibiotic rifampin, the antifungal griseofulvin, certain HIV medications, some anti-seizure drugs (which are also sometimes prescribed for bipolar disorder), and the herbal supplement St. John’s Wort. Other common antibiotics and antifungals don’t cause this problem.
Going Back to Regular Birth Control
You can start or restart hormonal birth control immediately after taking Plan B. The two don’t interfere with each other, so there’s no need to wait. However, your regular method won’t be fully effective right away. You’ll need to use condoms or avoid sex for seven days while the hormonal contraception builds up protection.
If you vomit within two hours of taking Plan B, the pill may not have been fully absorbed. In that case, it’s worth contacting a pharmacist or healthcare provider about whether you need another dose or an anti-nausea medication to keep it down.
How to Know If It Worked
There’s no immediate way to confirm that Plan B prevented pregnancy. The clearest sign is getting your period, even if it comes early, late, or looks different than usual. If three weeks pass after unprotected sex without any bleeding, take a home pregnancy test. Most standard tests are accurate from the first day of a missed period, and highly sensitive tests can sometimes detect pregnancy a few days before that.
Plan B is a single-dose intervention, not an ongoing method. It won’t protect you from pregnancy if you have unprotected sex again in the days or weeks after taking it. Each act of unprotected sex carries its own risk, and the pill only addresses the window around the time you took it.