How Long Is Plan B Effective After Taking It?

The short answer: within 72 hours, and the sooner the better

Plan B is labeled for use within 72 hours, which is three days, of unprotected sex, a window confirmed by FDA labeling. Its effectiveness decreases the longer you wait, so the single most important thing you can do to make it work is to take it as soon as possible. Every hour genuinely counts, and there is never an advantage to putting it off.

It can still offer some protection up to 120 hours, or five days, but it is much less effective in that later stretch, and it is most reliable in the first 72 hours, especially the first day. If more than three days have passed, two other methods are stronger: ella, a prescription pill that stays effective for a full five days, and the copper IUD, the most effective emergency option, which works up to five days regardless of timing. We will get to both of those options in detail a little further down.

And the second half of the question deserves a clear flag up front, because the phrase “effective after taking it” misleads a lot of people. Plan B is not a shield that stays up for the rest of your cycle. It works on the unprotected sex that already happened, and it does nothing for unprotected sex you have afterward. If you have sex again in the days after taking it, you are at risk again. We will explain exactly why later on.

The 72-hour window, hour by hour

Effectiveness is not a single number; it slides down as the hours pass. Studies estimate the highest effectiveness in the first day, with a clear decline across the three-day window. The figures below are rough estimates drawn from research, and reported ranges vary between studies, so treat them as a pattern rather than a promise.

The shape of that decline is the whole message. Taken within a day, Plan B prevents a large majority of the pregnancies that would otherwise have happened. By the back end of the window, its effectiveness has fallen substantially. Overall, taking it within three days is often described as reducing the chance of pregnancy by somewhere in the range of roughly 75 to 89 percent, but that headline number hides the fact that fast use sits well above it and slow use well below it.

This is why the advice is always the same: do not wait. There is no benefit to delaying and a real cost, because the longer you sit with it, the more the odds slip. If you have the pill, the best version of taking it is taking it now.

It is worth being honest about why the reported numbers vary so much from one source to another, because the spread can look confusing. Different studies enrolled people at different points in their cycles, measured outcomes in different ways, and defined the time windows slightly differently, so the exact percentages shift depending on the study. What does not shift is the direction: earlier is more effective, later is less, and the drop is steep enough that a delay of a day or two genuinely changes your odds. Rather than fixating on whether a given figure is exactly right, the safer reading is to treat any single number as approximate and let the overall slope guide your urgency.

What about after 72 hours?

Two things are true at once here, and both are worth holding. First, Plan B is officially labeled for 72 hours, and its effectiveness past that point is much lower. Second, it is not a hard cutoff where the pill suddenly does nothing; the American College of Obstetricians and Gynecologists notes that some studies suggest it can be moderately effective up to five days, with five days being the latest point at which it has been shown to reduce the chance of pregnancy at all.

So if it has been more than 72 hours, the guidance splits in a useful way. Do not assume Plan B is useless and skip it; it may still help, just with reduced effectiveness, so taking it is reasonable if it is what you can get. But also recognize that past 72 hours you have better choices, and it is worth pursuing them quickly.

The takeaway: 72 hours is the line where Plan B is at its best stretched as far as it reliably goes, and beyond it the pill becomes a weaker fallback while ella and the copper IUD become the front-runners. Either way, speed still helps, since all of these methods work better the sooner they are used.

Why sooner is always better: the ovulation connection

The decline over time is not arbitrary. It is tied directly to ovulation, which is the deeper reason behind every “take it as soon as possible” instruction.

Plan B works mainly by delaying or preventing ovulation. As the hours after unprotected sex tick by, ovulation, if it has not already happened, draws closer. The later you take the pill, the more likely it is that your body is about to ovulate or already has, and once ovulation occurs the pill can no longer prevent it. A frequently cited review found that women who took levonorgestrel on or after the day of ovulation had pregnancy rates similar to using no contraception at all, while none of those who took it before ovulation became pregnant. The time window and the cycle window are really the same story seen from two angles.

So when you see “sooner is better,” read it as “sooner gives you a better chance of beating ovulation.” That framing makes the urgency concrete instead of vague. You are racing a biological clock you cannot see, and the only safe assumption is that it is closer than you think.

This also explains why two people who took Plan B at the very same number of hours after sex can have different outcomes. One may have been days away from ovulation, giving the pill plenty of room to delay it, while the other may have been right on the edge, leaving no room at all. The hours on the clock were identical, but their positions in the cycle were not, and that hidden difference is what tipped the result. It is one more reason to act on the only lever you fully control, which is how fast you take it, and to stop second-guessing factors you cannot change in the moment.

What lowers Plan B’s effectiveness, besides time

Timing is the single biggest factor, as the hour-by-hour decline showed, but it is not the only one. A few other things can pull effectiveness down, and most of them have a simple workaround if you know to ask.

A closer look at weight

The weight question is the one people worry about most, so it is worth being precise and calm about it. There is no official weight limit for Plan B, and the FDA has not set one. However, older research suggests levonorgestrel pills may be less effective for people who weigh around 165 pounds or more, or who have a BMI of 30 or more. ella may be more effective than Plan B in the 165 to 194 pound range, while it in turn may be less effective at 195 pounds or above. The copper IUD sidesteps the whole issue, since it works regardless of body weight.

A closer look at medications

Certain medicines can reduce how well Plan B works, including some anti-seizure drugs, some treatments for other conditions, and the herbal supplement St. John’s wort. If you take regular medication, it is worth a quick word with a pharmacist, who can tell you whether your dose is likely to be affected and whether a copper IUD, which does not rely on a hormone reaching a certain level in your blood, would be a more reliable choice. None of this should slow you down; ask the question while you act. To see which option fits your situation, the emergency contraception calculator can help you weigh timing, weight, and method together.

How long does Plan B take to work?

People often expect a moment when the pill “kicks in,” and then feel anxious when nothing seems to happen. It helps to reset that expectation, because of how the pill actually works.

Plan B does its job by acting on your hormones to delay or prevent ovulation, a process that unfolds over the hours after you take it rather than in a single instant. There is no switch that flips, no sensation of it working, and no specific hour at which you can say it succeeded. The pill is quietly influencing whether and when an egg is released, which is not something you can feel happening.

So “how long does it take to work” does not have a stopwatch answer. The useful version is this: take it as soon as you can to give it the best chance, then wait for your period as the real signal, rather than scanning your body for a sign that the pill is not designed to give. Any spotting or cycle changes in between are about the hormones settling, not a status update, and we explain those in can Plan B mess up your cycle.

Does Plan B keep protecting you after you take it?

This is the second meaning of the question, and getting it wrong leads to unintended pregnancies, so it deserves a blunt answer: no. Plan B does not provide ongoing protection. It is a one-time backup for unprotected sex that already happened, not a contraceptive that guards the days ahead.

The reason follows from the mechanism. Plan B works by delaying a single ovulation. If you have unprotected sex again in the days after taking it, that delayed egg may still be released, and the new sperm can meet it, so the later encounter carries its own fresh risk that the earlier pill does nothing about. In fact, because the pill can push ovulation later in your cycle, having unprotected sex soon afterward can be a genuine pregnancy risk.

This is also why relying on emergency pills repeatedly leaves gaps that a regular method would close, a point we go through in taking Plan B too many times. The pill is excellent at the narrow job of covering one past event. Outside that job, it is not protection, and assuming otherwise is the mistake to avoid.

How long does Plan B stay in your body?

It is worth separating two ideas that sound alike but are different: how long the drug remains in your system, and how long it keeps protecting you. The first is a matter of pharmacology; the second, as we just covered, is essentially the moment it acts on, and no longer.

Levonorgestrel clears the body fairly quickly. Its half-life is roughly a day to a day and a half, and only trace amounts remain after about four days. So within a short window the active hormone is largely gone. That rapid clearance is part of why the pill does not offer lasting protection, there is simply not much of it left after a few days, and also part of why it does not affect your fertility going forward.

Holding these apart prevents a common error, the assumption that because the medicine is technically still around, you must still be protected. You are not. The drug clears in days, and its protective action ended at the single ovulation it was racing to delay.

The same distinction also clears up a worry that points the other way. Some people, having learned that the drug leaves quickly, fret that it left too soon to have worked. That is not how it functions either. The pill does its job in the short window while it is present, by acting on ovulation, and then it is no longer needed, so its quick exit is not a sign of weakness. Working briefly and then clearing is exactly what a single-use emergency medicine is supposed to do, and it is the same reason it has no lasting effect on your body or your future fertility.

Vomiting and other complications

A few practical wrinkles can affect whether your dose counts, and they are easy to handle if you know about them in advance.

None of these are reasons to panic; they are reasons to ask a question. A pharmacist can sort out almost all of them in a couple of minutes, and that conversation is free and routine. The cost of asking is tiny next to the cost of a dose that quietly did not count.

Plan B, ella, and the copper IUD: timing windows compared

Since this whole article is about timing, it helps to see how the three emergency options handle the clock differently. The table focuses on the timing angle; for a fuller head-to-head on the pills, see ella versus Plan B.

Read across the timing rows and a clear pattern emerges. Plan B is the easiest to get but the one whose effectiveness fades fastest as the hours pass. ella holds its effectiveness steady across the full five days, which makes it the stronger pill choice the later you are in the window. The copper IUD does not fade at all and is the most effective option throughout, with the trade-off that it needs a clinic visit to place.

So the right choice often depends on how much time has passed. Early on, any of them can work well, and Plan B’s easy access makes it a sensible pick. Later in the window, the steadier methods pull ahead. If you are unsure which still fits your timing, the Waldev calculator lays out which options remain effective for how long it has been, so you are not guessing.

Real-world scenarios

Here is how the timing plays out in common situations, with the practical read on each. These are illustrative examples for understanding, not medical advice for your specific situation.

The thread is simple: act fast on each event, do not treat one dose as lasting coverage, and once you pass 72 hours, lean toward the methods built for the five-day window. To see which options still fit your exact timing, run it through the calculator.

Common timing mistakes people make

A few avoidable mistakes cost people effectiveness or leave them unprotected. Each one comes from a reasonable-sounding assumption, and each is worth heading off.

Avoiding these comes down to two habits: act immediately rather than waiting for a tidy moment, and never treat a single dose as lasting coverage. Getting both right is most of what makes emergency contraception work as well as it can. If you want a quick read on your specific timing before you act, the free calculator gives you one in about a minute.

Frequently asked questions

Sources & references

This article is based on guidance from major medical bodies and peer-reviewed research. Effectiveness figures are estimates and vary by study; always confirm details with a current source or a clinician.