Plan B starts working almost immediately after you swallow it. The active hormone reaches peak levels in your bloodstream within about 1.5 to 2 hours, with a range of 1 to 4 hours depending on the person. But “working” in this case doesn’t mean you’ll feel anything happen. The pill quietly intervenes in your reproductive cycle behind the scenes, and the sooner you take it after unprotected sex, the more effective that intervention is.
What Plan B Does in Your Body
Plan B contains a synthetic hormone that your body absorbs quickly through your digestive tract. After a single dose, blood levels of the hormone peak at roughly 1.67 hours on average, based on FDA pharmacokinetic data from 30 healthy women. That means the pill is fully absorbed and circulating well before you’d notice any physical changes.
The primary way it prevents pregnancy is by stopping or delaying ovulation. If your ovary hasn’t released an egg yet, the hormone surge from Plan B can pause that process, closing the window where sperm could fertilize anything. This is why timing matters so much: if you’ve already ovulated, the pill has far less to work with. It does not end an existing pregnancy, and it won’t affect a fertilized egg that has already implanted in the uterus.
Why Every Hour Counts
Plan B reduces the chance of pregnancy by 75 to 89 percent when taken within 72 hours (3 days) of unprotected sex. But that range is misleading if you read it as a flat number. Effectiveness is highest in the first 24 hours and drops steadily from there. Taking it within the first 12 hours gives you the best odds.
You can technically take Plan B up to 5 days (120 hours) after unprotected sex, but its effectiveness drops noticeably after the 3-day mark. If you’re past 72 hours and still need emergency contraception, it’s worth knowing that a different type of emergency contraceptive pill (sold under the brand name ella) maintains higher effectiveness through day 5. A copper IUD, inserted by a provider within 5 days, is the most effective emergency option at any point in that window.
Side Effects and What to Expect
Most people don’t feel much after taking Plan B, but some experience nausea, fatigue, headache, or breast tenderness within the first few hours. These side effects are caused by the same hormonal surge that prevents ovulation, and they typically resolve within a day or two. Nausea is the most common complaint. If you vomit within 2 hours of taking the pill, your body may not have absorbed enough of the hormone for it to work, and you should take another dose.
Your next period may arrive a few days earlier or later than expected. Some people also notice heavier or lighter bleeding than usual, or spotting between periods. These cycle changes are normal and temporary. If your period is more than a week late, take a pregnancy test.
Body Weight and Effectiveness
There is a well-documented relationship between body weight and how well Plan B works. Effectiveness decreases as body weight increases. The Society of Family Planning has acknowledged this in clinical guidance, though the exact thresholds are still debated. Most evidence suggests that Plan B becomes less reliable for people weighing over 165 pounds, and it may offer minimal protection above 195 pounds.
If you fall into this range, ella or a copper IUD are more reliable alternatives. Neither loses effectiveness at higher body weights the way Plan B does.
How You’ll Know It Worked
There is no immediate confirmation that Plan B was effective. You won’t feel a specific sensation, and there’s no test you can take the next day. The only real signal is your period arriving roughly on schedule. If it comes within a week of when you expected it, the pill almost certainly worked. If it’s more than a week late, or if you experience unusual symptoms like severe lower abdominal pain, a pregnancy test is the next step.
Plan B is a single-dose intervention, not an ongoing form of birth control. It doesn’t protect you from pregnancy for the rest of your cycle. If you have unprotected sex again after taking it, even later that same week, you’re at risk again. Using a regular contraceptive method gives you far more reliable, ongoing protection than relying on emergency contraception repeatedly.