Plan B starts working fast. The active ingredient reaches its peak concentration in your bloodstream within about 1 to 2 hours of swallowing the pill, with an average of roughly 1.6 hours for the tablet form. But “kicking in” and “preventing pregnancy” aren’t quite the same thing, and understanding the difference matters.
What Happens in the First Two Hours
Plan B contains a synthetic hormone called levonorgestrel, which your body absorbs rapidly through the digestive tract. Within one to two hours, blood levels hit their peak and the drug begins its primary job: delaying or blocking the release of an egg from the ovary. If ovulation hasn’t happened yet, Plan B can suppress the hormonal surge that triggers it, postponing egg release by five or more days. That delay is long enough for sperm (which survive about five days inside the body) to die off before they ever encounter an egg.
This is why timing matters so much. Plan B is most effective when taken before ovulation occurs. If the egg has already been released, the pill has far less ability to prevent pregnancy.
Why Every Hour Counts
Plan B is roughly 94% effective when taken within the first 24 hours after unprotected sex. That number drops to about 58% by the 72-hour mark. The pill can technically be taken up to 120 hours (five days) afterward, but its effectiveness continues to decline past 72 hours, and other options work better in that later window.
The takeaway is simple: the sooner you take it, the better it works. There’s no benefit to waiting, and every hour of delay reduces your odds of preventing pregnancy.
What You Might Feel After Taking It
Because the drug is absorbed so quickly, side effects can show up within a few hours. The most common ones are nausea, headache, dizziness, and breast tenderness. These are generally mild and short-lived. Some people feel nothing at all.
One important detail: if you vomit within two hours of taking the pill, your body may not have absorbed the full dose. In that case, you likely need to take another pill. A pharmacist can confirm whether a second dose is necessary.
Weight and Effectiveness
Plan B becomes significantly less effective if you weigh more than 165 pounds. This doesn’t mean it won’t work at all, but the protection drops enough that an alternative is worth considering. A copper IUD, which can be placed by a healthcare provider within five days of unprotected sex, is the most effective form of emergency contraception regardless of body weight. Another pill option, ella (ulipristal acetate), also works better than Plan B in the 72-to-120-hour window and may be more effective at higher body weights.
What to Expect Afterward
Plan B can shift the timing of your next period. It might come a few days early, a few days late, or be heavier or lighter than usual. Some spotting between periods is also normal. These changes happen because the large dose of hormone temporarily disrupts your cycle, and things typically return to normal by the following month.
If your period is more than a week late, that’s a signal to take a pregnancy test. Even if your cycle seems roughly on schedule, the most reliable way to confirm Plan B worked is to take a pregnancy test three weeks after the day you took the pill. Standard home tests are accurate at that point.
Plan B Won’t Protect You Going Forward
Plan B is a single-event intervention, not ongoing contraception. It does nothing to prevent pregnancy from sex that happens after you take it. If you have unprotected sex again, even later the same day, you’re not covered. The pill also doesn’t protect against sexually transmitted infections.
If you find yourself reaching for Plan B more than occasionally, that’s a practical sign it might be worth exploring a regular contraceptive method that doesn’t require you to act after the fact.