Plan B (levonorgestrel 1.5 mg) is designed to be taken within 72 hours of unprotected sex, and its effectiveness drops steadily with every hour that passes. Within that window, it prevents roughly 87% to 90% of expected pregnancies, but the drug does not simply stop working at hour 73. Evidence shows it still offers meaningful protection between 72 and 120 hours, though at reduced rates. The “72-hour window” is the recommended timeframe for best results, not a hard cutoff after which the pill becomes useless.
Why Sooner Is Always Better
The single most important thing to know about Plan B is that time matters more than almost any other factor. The pill works primarily by delaying or preventing ovulation, so its effectiveness depends heavily on where you are in your cycle and how quickly the drug enters your system. Research consistently shows that the earlier you take it, the better it works. The pregnancy rate in a large analysis was about 0.8% when the pill was taken within 72 hours, compared to 1.8% for people who took it between 72 and 120 hours after unprotected intercourse.1PubMed. Effectiveness of emergency contraceptive pills between 72 and 120 hours after unprotected sexual intercourse That is still a substantial reduction in pregnancy risk compared to taking nothing, but the gap makes it clear that even a few hours of delay costs you protection.
Within the 72-hour window itself, effectiveness is not flat. Studies generally show the best results when the pill is taken within the first 24 hours, with a slight decline between 24 and 48 hours, and a further decline between 48 and 72 hours. If you have the pill on hand or can get it quickly, taking it immediately gives you the strongest odds. Waiting until the next morning or the next day because it feels less urgent is a real and avoidable loss of effectiveness.
What Actually Happens in Your Body
Plan B contains levonorgestrel, a synthetic form of progesterone. Its main job is to delay the release of an egg from the ovary. If the egg never meets a sperm cell, pregnancy cannot happen. This is why timing relative to ovulation is so critical: if you have already ovulated, the drug has little to work with. Research on the mechanism confirms that levonorgestrel-based emergency contraceptives are ineffective after ovulation has occurred.2PubMed. Mechanisms of action of hormonal emergency contraceptives
This is where a common misunderstanding arises. Many people assume Plan B works like an “after-the-fact” contraceptive that prevents a fertilized egg from implanting in the uterus. The evidence points strongly toward ovulation suppression as the primary mechanism, not interference with implantation. If ovulation has already happened, the drug is unlikely to prevent pregnancy regardless of when you take it within the 72-hour window. This distinction matters because it means Plan B is not an early abortion pill. It prevents pregnancy from starting in the first place, provided the timing lines up.
After 72 Hours, It Still Works, Just Less Well
The 72-hour label on the box is conservative guidance, not a biological cliff edge. The same study that tracked outcomes between 72 and 120 hours found that emergency contraceptive pills reduced the risk of pregnancy even in that later window, with effectiveness ranging from about 72% to 87% of expected pregnancies prevented.1PubMed. Effectiveness of emergency contraceptive pills between 72 and 120 hours after unprotected sexual intercourse Those numbers are lower than the under-72-hour range, but they are not zero. If you are past the three-day mark and have no other option available, taking Plan B is still better than doing nothing.
That said, if you are in the 72-to-120-hour range, you have a stronger alternative available in most cases: a prescription pill called ella (ulipristal acetate), which is specifically approved for use up to 120 hours and maintains better effectiveness across those five days. More on that below.
The Fertile Window and Sperm Survival
To understand why Plan B sometimes fails even when taken on time, it helps to know what is happening on the other side of the equation. Sperm cells do not die immediately after sex. Research on sperm survival in the human reproductive tract has demonstrated that sperm can remain viable in the fallopian tubes for roughly 85 hours, which is about three and a half days.3Gynecologic and Obstetric Investigation. Sperm Transport to and Survival in the Human Fallopian Tube Earlier research established that the window from about five days before ovulation through two days after ovulation represents the period when pregnancy is most likely.4PubMed. Sperm distribution and degradation in the human female reproductive tract
This means that if you have unprotected sex a few days before ovulation, sperm can be sitting in the fallopian tubes waiting for an egg. Plan B’s job is to delay that egg long enough for the sperm to die off. But if ovulation is imminent or has already begun by the time the drug kicks in, the delay may not be enough. The interplay between sperm lifespan, your ovulation timing, and when you take the pill is why emergency contraception works well on average but is never a guarantee for any individual person.
Does Body Weight Affect How Well It Works
This is one of the most talked-about questions around Plan B, and the evidence is genuinely complicated. In 2013, European drug authorities updated the label on a levonorgestrel emergency contraceptive product to warn that effectiveness may be reduced in people with higher body mass index, which generated widespread media attention in the United States.5PubMed Central. “My BMI is too high for Plan B.” A changing population of women seeking ulipristal acetate emergency contraception online The concern makes pharmacological sense: levonorgestrel is distributed through body tissue, so a larger body may dilute the drug’s concentration.
Pharmacokinetic research bears this out to a degree. A study comparing drug levels in people with obesity versus those at a normal BMI found that the peak blood concentration of levonorgestrel was meaningfully lower in the higher-weight group when taking a standard single dose. When the dose was doubled, the peak levels in the higher-weight group rose to approximate what normal-BMI individuals achieve on a single dose.6PubMed Central. Impact of obesity on the pharmacokinetics of levonorgestrel-based emergency contraception: single and double dosing That sounds like a tidy solution: just take two pills. But a clinical trial testing exactly that approach found no meaningful difference in the key outcome. Roughly half of participants in both the standard-dose and double-dose groups went at least five days without evidence of ovulation, and the difference was not statistically significant.7PubMed Central. Double Dosing Levonorgestrel-Based Emergency Contraception for Individuals With Obesity: A Randomized Controlled Trial
So where does that leave you? Getting more drug into the bloodstream did not translate into better ovulation suppression in that trial. The honest answer is that Plan B may be somewhat less effective at higher body weights, but the evidence is not strong enough to say it fails entirely or that doubling up reliably fixes the problem. For people with a higher BMI who want the most effective emergency contraception, the copper IUD or ella are generally better-studied options, which brings us to what else is available.
Alternatives When Plan B Is Not the Best Fit
Plan B is the most widely recognized emergency contraceptive in the United States, but it is not the only one, and in some situations it is not the best one.
Ella (ulipristal acetate): This prescription-only pill works through a different mechanism than Plan B. It is a selective progesterone receptor modulator that can delay ovulation even when the hormonal surge triggering ovulation has already started, a point at which levonorgestrel loses most of its effectiveness. Ella is approved for use up to 120 hours (five days) after unprotected sex, and research indicates it is more effective than levonorgestrel-based pills at one, three, and five days post-intercourse.8PubMed Central. “The Difference between Plan b and ella®? They’re Basically the Same Thing”: Results from a Mystery Client Study The tradeoff is that ella requires a prescription in the U.S. (though telehealth has made this easier), and you cannot use it alongside hormonal birth control pills for a few days because the two drugs can interfere with each other.
Copper IUD: The most effective form of emergency contraception is a copper intrauterine device inserted within five days of unprotected sex. In a recent study, no pregnancies occurred among 318 participants who received a copper IUD after unprotected intercourse, preventing 100% of expected pregnancies.9PubMed Central. Estimating emergency contraception efficacy with levonorgestrel and copper intrauterine devices The copper IUD also doubles as long-term contraception for up to ten or more years, which makes it an appealing option for people who also want ongoing protection.
Levonorgestrel IUD: A newer finding is that the hormonal levonorgestrel IUD (the 52 mg version used in products like Mirena or Liletta) also appears to work as emergency contraception. In the same study, only one pregnancy occurred among 312 participants who received a levonorgestrel IUD, preventing roughly 94% to 96% of expected pregnancies.9PubMed Central. Estimating emergency contraception efficacy with levonorgestrel and copper intrauterine devices This is a significant finding because the levonorgestrel IUD is often preferred for long-term use due to lighter periods and fewer side effects than the copper IUD. However, this use is still relatively new in the research, and guidelines are evolving.
Drug Interactions That Can Undermine Plan B
One underappreciated risk is that certain medications can dramatically reduce how much levonorgestrel your body absorbs. The clearest example comes from research on efavirenz, an antiretroviral drug used to treat HIV. In a pharmacokinetic study, efavirenz cut the total levonorgestrel exposure by about 56% and the peak blood concentration by about 41%.10PubMed Central. Pharmacokinetic interactions between the hormonal emergency contraception, levonorgestrel (Plan B), and Efavirenz That is a massive reduction that could plausibly render the drug ineffective.
Efavirenz is one example, but the broader concern extends to other medications that speed up the liver enzymes responsible for breaking down levonorgestrel. Some anti-seizure drugs, certain antibiotics (particularly rifampin), and the herbal supplement St. John’s Wort all have similar enzyme-inducing effects. If you take any of these regularly, Plan B may not reach the blood levels needed to delay ovulation. In those situations, a copper IUD is the most reliable emergency option because its mechanism does not depend on drug absorption at all.
Does Plan B Cause Ectopic Pregnancy
A persistent myth is that taking emergency contraception raises your risk of an ectopic pregnancy, where a fertilized egg implants outside the uterus, usually in a fallopian tube. A systematic review looked at this question directly and found that among 307 pregnancies of known location that occurred after levonorgestrel treatment, only three (about 1%) were ectopic.11PubMed Central. Ectopic Pregnancy and Emergency Contraceptive Pills: A Systematic Review That rate is consistent with the general background rate of ectopic pregnancies in the broader population, so there is no evidence that Plan B increases this risk.
The confusion may stem from the fact that if Plan B fails and pregnancy does occur, any pregnancy carries a small baseline chance of being ectopic. But the drug itself does not push a pregnancy toward that outcome. If you take Plan B and later experience sharp pelvic pain, dizziness, or unusual bleeding, seek medical attention, but those symptoms warrant evaluation regardless of whether you have taken emergency contraception.
Common Timing Mistakes and Practical Realities
Knowing that Plan B works best within 72 hours is only useful if you can actually get it within that window. Plan B is available over the counter without age restrictions in the United States, but access can still be uneven. Some pharmacies keep it behind the counter or in locked cases, and not every pharmacy stocks it. In rural areas, the nearest retailer may be a significant drive away. Online ordering is an option, but shipping times can eat into that 72-hour window.
One practical strategy is to keep a dose on hand before you need it, the same way you might keep a first-aid kit in your home. Plan B has a shelf life of several years when stored at room temperature, and having it available eliminates the biggest barrier to timely use. If you are someone who uses condoms as your primary method, having a backup dose means a condom failure does not turn into a scramble at midnight.
Another common mistake is assuming that taking Plan B means you are protected for the rest of your cycle. You are not. The pill addresses one specific ovulation event. If you have unprotected sex again a few days later, you are at risk again. You can take Plan B more than once in a cycle if needed, but it should not become a routine method. Regular contraception is far more effective and less disruptive to your cycle.
Side Effects and What to Expect Afterward
Plan B can cause nausea, headache, fatigue, breast tenderness, and spotting or heavier bleeding. These side effects are generally mild and resolve within a day or two. Nausea is the most commonly reported one, and it is more likely if you take the pill on an empty stomach. Eating a small meal beforehand can help.
Your next period may come earlier or later than expected. If it arrives more than a week late, take a pregnancy test. A late period does not necessarily mean Plan B failed; the hormonal disruption caused by levonorgestrel can shift your cycle by several days. But a very late period combined with other early pregnancy symptoms warrants a test to be sure.
There are no known long-term health effects from taking Plan B, even multiple times. It does not affect your future fertility. The hormonal dose is larger than what is in a daily birth control pill, which is why the side effects can feel more intense in the short term, but the drug clears your system quickly. People sometimes worry about taking it “too many times,” but the concern there is more about effectiveness and cost than safety. If you are reaching for Plan B frequently, it is worth exploring a more reliable ongoing method.
When Plan B Cannot Help
Because Plan B’s mechanism centers on delaying ovulation, there are situations where it simply cannot work. If you have already ovulated within the past day or so, the egg is already in the fallopian tube, and no amount of levonorgestrel will call it back. The drug also cannot end an existing pregnancy. If a fertilized egg has already implanted in the uterine lining, Plan B will not dislodge it. This is fundamentally different from medications like mifepristone, which are used in medical abortion.
The inability to know exactly when you ovulated is one of the frustrating realities of emergency contraception. Unless you are tracking your cycle with detailed methods like basal body temperature or ovulation predictor kits, you are making your best guess. This uncertainty is another reason why taking the pill as soon as possible matters so much: the sooner you act, the more likely it is that ovulation has not yet occurred, and the better the drug can do its job.