Plan B is 81% to 90% effective at preventing pregnancy, depending on how quickly you take it after unprotected sex. That’s a strong reduction in risk, but it’s not a guarantee. The sooner you take it, the better it works, and several factors like your weight, where you are in your menstrual cycle, and certain medications can shift those numbers significantly.
How Timing Changes Effectiveness
Plan B works best when taken within the first 24 hours after unprotected sex. With each passing day, the pill becomes less reliable. The standard window is 72 hours (three days), but you can still take it up to 120 hours (five days) after sex with reduced effectiveness.
The most optimistic estimates put Plan B at up to 90% effective when used promptly. However, more rigorous analyses have found lower numbers. Two separate meta-analyses calculated effectiveness rates of 72% and 77%, respectively. The gap between these figures and the 90% number reflects differences in study design and how “effectiveness” is measured, but the takeaway is consistent: Plan B substantially lowers your chance of pregnancy, though it doesn’t eliminate it. Out of every 100 women who use Plan B after a single act of unprotected sex, somewhere between 1 and 3 will still become pregnant.
How Plan B Actually Works
Plan B contains a synthetic hormone called levonorgestrel. Its primary job is to delay or prevent your ovary from releasing an egg. Think of it as pulling an emergency brake on ovulation. If no egg is released, sperm have nothing to fertilize, and pregnancy can’t happen.
This mechanism is also why timing matters so much. If your body has already started ovulating, Plan B won’t work. The pill cannot interrupt a pregnancy that has already begun, and it does not affect a fertilized egg that has already implanted. It is not an abortion pill. Its entire strategy depends on stopping ovulation before it happens, which is why taking it as early as possible gives you the best odds.
Why Body Weight Matters
One of the most important and underreported limitations of Plan B is that it becomes significantly less effective at higher body weights. Research from Oregon Health & Science University found that people with a BMI of 30 or higher experienced Plan B failure four times as often as those with a BMI under 25. The reason is straightforward: blood levels of levonorgestrel were about 50% lower in individuals with a BMI of 30 after taking a standard dose. The hormone simply never reaches the concentration needed to reliably block ovulation.
Doubling the dose doesn’t solve the problem either. A randomized controlled trial enrolling 70 individuals with BMIs above 30 and weights of at least 176 pounds found that a double dose of Plan B was still not effective at preventing pregnancy in this group. If you’re in this weight range, a different type of emergency contraception is a better option (more on that below).
Plan B vs. Ella
Ella is a prescription emergency contraceptive that uses a different active ingredient. Within the standard 72-hour window, clinical trials show pregnancy rates of roughly 0.9% to 1.8% for ella compared to 1.7% to 2.6% for Plan B. Those differences are modest, and researchers haven’t conclusively established that one outperforms the other within that first three-day window.
Where ella has a clearer advantage is in two situations. First, it works closer to the time of ovulation than Plan B does, meaning it’s more likely to be effective if you’re later in your cycle. Second, it maintains its effectiveness better across the full five-day (120-hour) window. In one large study, pregnancy rates over the full 120-hour period were 1.6% for ella users versus 2.6% for Plan B users. If more than three days have passed, or if you suspect you may be close to ovulating, ella is the stronger choice. It does require a prescription, unlike Plan B.
The Copper IUD Option
The most effective form of emergency contraception isn’t a pill at all. A copper IUD, inserted by a healthcare provider within five days of unprotected sex, prevents fertilization by creating a chemical environment hostile to both sperm and egg. It’s over 99% effective as emergency contraception, and once it’s in place, it continues working as long-term birth control for up to 10 years. The tradeoff is that it requires a clinic visit and insertion procedure, which isn’t always accessible on short notice. But if effectiveness is your top priority and you can get an appointment quickly, no pill matches it.
Medications That Reduce Effectiveness
Several drugs can interfere with how well Plan B works by speeding up the rate at which your body breaks down levonorgestrel. These include certain anti-seizure medications, some antibiotics, anti-HIV drugs, and the herbal supplement St. John’s Wort. If you take any of these regularly, Plan B may not reach the hormone levels needed to block ovulation. Talk to a pharmacist about whether an alternative emergency contraceptive would be more appropriate.
What to Expect Afterward
Plan B commonly causes your next period to arrive earlier or later than expected. Some people experience spotting, nausea, headaches, or breast tenderness in the days after taking it. These side effects are temporary and result from the burst of synthetic hormone. If your period is more than a week late, it’s worth taking a pregnancy test.
Plan B is available over the counter without a prescription or ID. Brand-name Plan B One-Step typically costs $40 to $50, while generic versions can be as low as $11. You may need to ask for it at the pharmacy counter, but no one can legally require you to show identification or prove your age to buy it.
Putting the Numbers in Perspective
It helps to understand what “81% to 90% effective” actually means in context. These figures don’t mean you have a 10% to 19% chance of getting pregnant after taking Plan B. They represent how much Plan B reduces the risk of pregnancy that would have existed without it. If 8 out of 100 women would have become pregnant from a single act of unprotected sex during a fertile window, Plan B might reduce that to 1 or 2 out of 100. For encounters that happened outside the fertile window, the baseline risk was already low, and Plan B lowers it further.
The practical bottom line: Plan B is a genuinely effective backup, especially when taken within the first 24 hours. But it’s less reliable if you weigh over 176 pounds, if you’ve already started ovulating, or if you wait several days to take it. In those cases, ella or a copper IUD offer meaningfully better protection.