Plan B starts working almost immediately after you swallow it. The active ingredient, levonorgestrel, is absorbed through your digestive system and reaches its peak levels in your blood within about 1.5 to 2 hours. But “kicking in” and “preventing pregnancy” aren’t quite the same thing, and understanding the difference matters if you’re trying to figure out whether the pill will work for you.
How Plan B Prevents Pregnancy
Plan B works by delivering a large dose of a synthetic hormone that delays or blocks ovulation. Your body normally releases an egg from the ovary after a surge of luteinizing hormone (LH). Plan B interrupts that hormonal surge, keeping the egg from being released in the first place. If there’s no egg, sperm have nothing to fertilize.
This is why timing matters so much. Plan B is effective when taken before the LH surge has begun. Two well-designed studies found that women who took it on the day of ovulation or after became pregnant at exactly the rate you’d expect with no contraception at all. In other words, once ovulation has already happened, Plan B does not appear to work. It prevents pregnancy by stopping the process before it starts, not by interfering with anything afterward.
Why Taking It Sooner Makes a Big Difference
Plan B can be taken up to 72 hours (three days) after unprotected sex, and some data supports partial effectiveness out to 120 hours (five days). But the numbers shift dramatically depending on how fast you act.
In a large study comparing time intervals, the pregnancy rate was 0.8% for women who took emergency contraception within 72 hours, compared to 1.8% for those who took it between 72 and 120 hours. That translates to an effectiveness rate of 87% to 90% in the first window, dropping to 72% to 87% in the later one. The World Health Organization recommends taking it as early as possible, ideally well before the 72-hour mark. Every hour you wait is an hour closer to a potential LH surge and ovulation.
What You Might Feel Afterward
Because the pill reaches peak blood levels quickly, side effects can show up within the first several hours. The most common ones include nausea, headache, dizziness, fatigue, breast tenderness, and cramping in your lower abdomen. Not everyone experiences these, and they’re typically mild and short-lived.
One side effect worth watching for: vomiting. If you throw up within two hours of taking Plan B, your body may not have absorbed enough of the drug. In that case, you should take another dose. After the two-hour mark, enough has been absorbed that vomiting is unlikely to affect how well it works.
You may also notice changes to your next period. It could arrive a few days early or late, and you might have light spotting in the days after taking the pill. This is a normal hormonal response and not a sign that something went wrong.
Body Weight Can Affect How Well It Works
Your weight plays a role in how effectively the standard 1.5 mg dose works. Reproductive health guidelines in the UK suggest that women weighing over 70 kg (about 154 pounds) or with a BMI over 26 consider taking a double dose (3 mg). The American Society for Emergency Contraception recommends the same double-dose approach for women with a BMI over 30.
If you fall into these ranges and are concerned about effectiveness, a copper IUD is another emergency contraception option that works regardless of body weight and can be placed up to five days after unprotected sex. Another pill-based alternative, ulipristal acetate (sold as ella), is more effective than Plan B in the 72- to 120-hour window, though it does require a prescription.
Medications That Can Reduce Effectiveness
Certain medications speed up how fast your liver breaks down levonorgestrel, which can lower the amount circulating in your body when you need it most. The most notable ones include the antibiotic rifampin, the herbal supplement St. John’s wort, and several anti-seizure medications like carbamazepine and phenobarbital. If you take any of these, a double dose of 3 mg is recommended to compensate for the faster breakdown.
This interaction is significant enough that if you’re on one of these drugs and need emergency contraception, it’s worth considering a non-hormonal option like the copper IUD instead, since its effectiveness isn’t influenced by other medications.
How to Know If It Worked
There’s no immediate signal that tells you Plan B successfully prevented pregnancy. The pill works silently by suppressing a hormonal process, and you won’t feel ovulation being blocked. The only reliable confirmation is getting your next period. If your period is more than a week late, a pregnancy test is a reasonable next step.
Keep in mind that Plan B is not a guarantee. Even under ideal conditions (taken within 24 hours, before ovulation, at a healthy weight), it reduces the risk of pregnancy significantly but does not eliminate it entirely. It’s a backup, not a primary method of contraception.