Plan B delays or prevents ovulation, and most people experience mild side effects like nausea and fatigue that resolve within a day or two. It’s most effective when taken as soon as possible after unprotected sex, with efficacy ranging from 81% to 90% depending on timing. Here’s what’s actually happening in your body and what to expect afterward.
How Plan B Works in Your Body
Plan B contains a synthetic hormone called levonorgestrel, which is the same type of progestin found in many daily birth control pills, just at a higher dose. Its primary job is to stop or delay the release of an egg from your ovary. If your body hasn’t released an egg yet, sperm have nothing to fertilize, and pregnancy doesn’t happen.
The FDA has concluded that Plan B has no direct effect on fertilization or implantation. It works entirely by disrupting ovulation and the hormonal surge that triggers it. This also means Plan B will not end an existing pregnancy. If a fertilized egg has already implanted in your uterus, the pill does nothing to change that. Plan B is not the same medication as the abortion pill, which uses completely different drugs (mifepristone and misoprostol) to end an established pregnancy. Emergency contraception, including Plan B, is legal throughout the United States.
Side Effects You Might Notice
Because Plan B delivers a concentrated dose of hormone, your body often reacts in ways that feel like amplified PMS. Common side effects include nausea, fatigue, headache, dizziness, breast tenderness, and lower abdominal pain. Most of these fade within a day or two.
Nausea is the side effect people worry about most, and for good reason: if you vomit within two hours of taking the pill, you need to take another dose because your body may not have absorbed enough of the medication. If nausea is a concern, eating a small snack before taking the pill can help, and taking it with water rather than on an empty stomach makes a difference for some people.
Changes to Your Next Period
Plan B commonly shifts the timing and character of your next menstrual cycle. Your period may arrive up to a week earlier or later than expected, and the flow can be lighter or heavier than usual. Some people also notice light spotting or bleeding in the days after taking the pill, well before their period is due. These irregularities are temporary and typically resolve by the following cycle.
If your period is more than three weeks late after taking Plan B, take a pregnancy test. A late period doesn’t automatically mean the pill failed, but it’s worth confirming.
How Timing Affects Effectiveness
Plan B is 81% to 90% effective at preventing pregnancy, and that range depends almost entirely on how quickly you take it. The sooner after unprotected sex you take the pill, the better it works. It can be taken up to 72 hours (three days) afterward, and some evidence supports reduced effectiveness out to 120 hours (five days), though waiting that long drops efficacy significantly.
There’s a critical limitation: Plan B cannot delay an ovulation that’s already happening. If you’ve already ovulated, the pill is far less likely to prevent pregnancy because the egg is already available for fertilization. The challenge is that most people don’t know exactly when they ovulate, so there’s no reliable way to tell in the moment whether you’re in that window. This is why speed matters so much. Taking it within the first 24 hours gives you the best chance.
Body Weight Can Reduce Effectiveness
Research shows that Plan B becomes less effective as body weight increases. One study found that pregnancy risk rose sharply starting around 70 to 75 kilograms (roughly 154 to 165 pounds), reaching a pregnancy rate of about 6% at 80 kilograms (176 pounds), compared to 1.4% for people in the 65 to 75 kilogram range. BMI showed a similar pattern, with effectiveness dropping noticeably around a BMI of 26.
If you weigh more than about 165 pounds, Plan B may still offer some protection, but it’s worth knowing that a copper IUD inserted within five days of unprotected sex is the most effective form of emergency contraception regardless of weight. Another option, ella (ulipristal acetate), also works by delaying ovulation and may maintain effectiveness at higher body weights better than levonorgestrel, though it requires a prescription.
No Impact on Future Fertility
Taking Plan B, even multiple times, does not affect your ability to get pregnant in the future. A systematic review of 22 studies looking at repeated use of emergency contraception found that the most common consequence was temporary menstrual irregularities, not fertility problems. The hormone leaves your system quickly, and your normal ovulation cycle resumes within days to weeks.
That said, Plan B isn’t designed to be your regular method of birth control. It’s less effective than daily contraceptive pills, IUDs, or implants, and the side effects are more noticeable because of the larger hormone dose. It works well as a backup, but ongoing contraception gives you much better odds month to month.