Plan B works within a specific window: you have up to 72 hours (three days) after unprotected sex for the best results, though it can still work up to 120 hours (five days). It does not provide any ongoing protection against pregnancy after that single event. The drug itself clears your body within a few days, with a half-life of roughly 27.5 hours, meaning most of it is gone within two to three days.
The Effectiveness Window, Hour by Hour
Plan B’s effectiveness drops significantly with every day you wait. Data from the FDA’s review of the original clinical trial breaks it down clearly:
- Within 24 hours: 95% of expected pregnancies prevented (pregnancy rate of 0.4%)
- 25 to 48 hours: 85% of expected pregnancies prevented (pregnancy rate of 1.2%)
- 49 to 72 hours: 61% of expected pregnancies prevented (pregnancy rate of 2.7%)
After 72 hours, effectiveness continues to decline. Some evidence supports partial effectiveness out to 120 hours, but at that point you’re well past the ideal window. The takeaway is simple: the sooner you take it, the better it works. Taking it within 24 hours nearly eliminates the risk.
How Long It Stays in Your Body
Plan B contains a synthetic hormone called levonorgestrel. After a single 1.5 mg dose, it has a half-life of about 27.5 hours, meaning half the drug is cleared roughly every day and a half. Within about five to six days, the hormone has been almost entirely metabolized and eliminated. This is why Plan B doesn’t provide lingering contraceptive protection. It delivers a short, concentrated burst of hormone that delays or prevents ovulation for that cycle, then it’s gone.
It Only Covers One Event
A common misconception is that Plan B keeps working for the rest of your cycle. It does not. The pill works by delaying or stopping ovulation for the specific window surrounding the unprotected sex that prompted you to take it. If you have unprotected sex again days later, even in the same cycle, you are not protected. You would need to take another dose or use a different form of contraception. Plan B is a one-time intervention, not a short-term birth control method.
How Long Side Effects Last
Most people who take Plan B experience mild side effects that resolve on their own. Common ones include nausea, fatigue, headaches, dizziness, breast tenderness, abdominal cramping, and light spotting. For most people, nausea and headaches fade within a day or two. Spotting can persist for several days and is not a sign that something is wrong.
The most noticeable lasting effect is a change to your next period. Your cycle may arrive earlier or later than expected. A period that’s a week late is not unusual after taking Plan B, since the pill works by disrupting your normal ovulation timing. If your period is more than a week late, a pregnancy test is reasonable. Wait at least two weeks after the unprotected sex for the most accurate result, since testing too early can produce a false negative.
Weight Can Reduce How Well It Works
Plan B becomes less effective at higher body weights. Clinical guidelines from the Faculty of Sexual and Reproductive Healthcare indicate that effectiveness starts to decline at a BMI above 26 or a weight above about 155 pounds (70 kg). This doesn’t mean it won’t work at all, but the failure rate increases. A different type of emergency contraceptive pill (sold under the brand name ella) retains better effectiveness at higher weights, though it too shows reduced efficacy above a BMI of 30 or roughly 187 pounds (85 kg). A copper IUD, inserted within five days, is the most effective emergency contraception regardless of weight.
Medications That Shorten Its Effectiveness
Certain medications speed up how quickly your liver breaks down levonorgestrel, which can reduce Plan B’s effectiveness before it has a chance to work. The main culprits are drugs that ramp up liver enzyme activity, including some medications used to treat epilepsy (such as carbamazepine and phenytoin), tuberculosis drugs like rifampicin, certain HIV medications (efavirenz alone reduces levonorgestrel levels by about 50%), and the antifungal griseofulvin. St. John’s wort, an herbal supplement commonly used for mood support, also lowers levonorgestrel levels. If you take any of these regularly, a copper IUD is a more reliable emergency option.
How It Actually Works
Plan B’s primary mechanism is straightforward: it delivers a high dose of synthetic progesterone that signals your body to delay or suppress ovulation. If your ovary hasn’t released an egg yet, sperm that are waiting in the reproductive tract have nothing to fertilize, and they die off within about five days. This is why timing matters so much. If ovulation has already occurred before you take the pill, Plan B is far less likely to prevent pregnancy. It does not interrupt an existing pregnancy or affect a fertilized egg that has already implanted.
You can take Plan B at any point in your menstrual cycle, but its effectiveness depends heavily on where you are relative to ovulation. If you’re in the early part of your cycle, well before ovulation, the pill has more room to work. If you’re right at the point of ovulation or just past it, the window has largely closed regardless of how quickly you took the pill.