How Effective Is ellaOne, and When Does It Fail?

EllaOne (ulipristal acetate) prevents pregnancy after unprotected sex in roughly 85% or more of cases when taken within its five-day window, making it the most effective emergency contraceptive pill available. But that headline number hides real variation depending on when in your cycle you take it, your body weight, and whether you have additional unprotected sex afterward. Understanding where ellaOne works well and where it falls short can help you make a more informed choice in a time-pressured situation.

How ellaOne Works

EllaOne’s active ingredient, ulipristal acetate, is a selective progesterone receptor modulator. In plain terms, it blocks the hormone progesterone from doing its job at key moments in your menstrual cycle. Its primary trick is delaying or preventing ovulation, the release of an egg from the ovary. It can postpone the surge of luteinizing hormone that triggers ovulation, and it may even suppress follicular rupture (the physical release of the egg) when that hormonal surge has already begun.¹ If there is no egg available to meet sperm, fertilization cannot happen. This ability to intervene even after the hormonal cascade has started is what gives ellaOne an edge over levonorgestrel (the active ingredient in most other morning-after pills), which loses its ovulation-blocking power once the LH surge is underway.

The Five-Day Window and Why Timing Still Matters

EllaOne can be taken up to 120 hours (five days) after unprotected intercourse, a longer window than levonorgestrel’s recommended 72 hours.² That five-day span is a genuine advantage for anyone who cannot get to a pharmacy quickly. But “up to five days” does not mean the pill works equally well on day one and day five. Taking it sooner is always better. The mechanism depends on catching ovulation before it happens, and every hour that passes is an hour closer to egg release.

There is an important nuance here that clinical data makes clear. EllaOne’s ability to block ovulation drops sharply as you approach the moment of egg release. One analysis found that its anti-ovulatory effect becomes negligible, around 8%, when taken roughly 36 hours before ovulation, which coincides with the most fertile days of the cycle.³ So the pill’s usefulness is tightly linked to where you are in your cycle at the moment you take it, not just how many hours have passed since sex.

This creates a practical problem: most people do not know exactly when they are going to ovulate. Without tracking basal body temperature or using ovulation predictor kits, the timing is a guess. EllaOne performs best when taken early in the fertile window, before the LH surge gains momentum. If ovulation has already occurred, the picture changes dramatically.

After Ovulation, ellaOne Offers Little Protection

This is the single most important scenario where ellaOne fails. If you have already ovulated by the time you take the pill, it cannot pull the egg back. Research examining whether ulipristal acetate works after ovulation found that it was no more effective at preventing pregnancy than chance alone in that situation, and it did not increase miscarriage rates either.⁴ In other words, if the egg is already out and fertilization has happened, ellaOne does not interrupt an established pregnancy. It simply has no meaningful effect at that point.

This finding has two practical implications. First, ellaOne is not an “abortion pill” and does not work like one. Second, if you are concerned that you may have ovulated around the time of unprotected sex, the pill’s protection is unreliable, and you should consider a copper IUD insertion as a more dependable backup. A copper IUD can be placed up to five days after unprotected intercourse and is the most effective form of emergency contraception available, regardless of where you are in your cycle.⁵

Body Weight and Reduced Effectiveness

Your weight affects how well ellaOne works, and this is one of the most underappreciated failure scenarios. Pooled data from phase III trials showed that the proportion of pregnancies was higher among obese women. Among non-obese women who did not have further unprotected sex in the same cycle, about 1.3% became pregnant. Among obese women who did have additional unprotected intercourse, the figure climbed to 8.3%, though that latter number came from a very small group of just 12 women.⁶

A separate analysis of randomized trial data confirmed that the risk of pregnancy was more than three times greater for obese women compared to those with a normal BMI, regardless of which emergency contraceptive pill they took. The odds ratio was about 3.6 overall. For levonorgestrel users, obesity made things even worse, with an odds ratio around 4.4. EllaOne fared somewhat better in obese women, with an odds ratio of about 2.6, though that result did not reach statistical significance.⁷ The practical takeaway: ellaOne is still a better bet than levonorgestrel for people at higher body weights, but its protection is meaningfully reduced compared to what it offers at lower weights. Again, a copper IUD sidesteps this issue entirely because its effectiveness does not depend on drug absorption or metabolism.

Having More Unprotected Sex in the Same Cycle

This is a failure scenario people often overlook. EllaOne is designed to address a single episode of unprotected intercourse by delaying ovulation. If you have unprotected sex again in the same cycle after taking the pill, you are essentially reintroducing sperm into a situation where ovulation may not have been permanently prevented, only delayed. The pooled trial data showed that subsequent unprotected intercourse in the same cycle was one of the strongest predictors of pregnancy among ellaOne users.⁶

If you take ellaOne and then have sex again without a barrier method, the delayed egg could still be released and meet fresh sperm. This is why guidance consistently recommends using condoms or another barrier for the remainder of the cycle after taking emergency contraception. It is also worth knowing that ellaOne can interfere with hormonal contraception: if you restart the pill, patch, or ring immediately after taking ellaOne, the two drugs may work against each other because they compete for the same progesterone receptors. The usual recommendation is to wait at least five days before resuming hormonal contraception and use barriers in the interim.

Side Effects Are Mild but Real

Emergency contraceptive methods in general, including ellaOne, levonorgestrel pills, and the copper IUD, carry only mild side effects, and serious adverse events are essentially unheard of.⁸ That said, you should know what to expect. Postmarketing surveillance covering more than a million women who used ellaOne found that the most frequently reported side effects were nausea, abdominal pain, and vomiting, along with headache, dizziness, irregular bleeding, delayed periods, and breast tenderness.⁹

The vomiting question deserves special mention. If you vomit within three hours of taking ellaOne, the pill may not have been fully absorbed, and you may need another dose. This is not a common occurrence, but it is worth being aware of, particularly if you are prone to nausea. Taking the pill with a small amount of food can help, though it is not strictly required.

Your next period may come earlier or later than expected after taking ellaOne. A delay of up to a week is considered normal. If your period is more than seven days late, taking a pregnancy test is a reasonable step. The drug does not cause long-term changes to your fertility or menstrual cycle.

How ellaOne Compares to Other Emergency Options

The hierarchy of emergency contraception effectiveness is well established: the copper IUD is the most effective option, followed by ulipristal acetate (ellaOne), with levonorgestrel pills coming in third.⁵ In real-world terms, here is what those differences mean for you:

Copper IUD: Pregnancy rates under 0.1% when inserted within five days. Works regardless of cycle timing or body weight. Requires a clinic visit and insertion by a healthcare provider, which limits accessibility. Also doubles as ongoing contraception for up to ten years.
EllaOne (ulipristal acetate): Effective up to 120 hours. Better than levonorgestrel in the 72-to-120-hour window and in women with higher BMI. Requires a prescription in some countries, available over the counter in others.
Levonorgestrel pills: Best within 72 hours, effectiveness drops off more steeply after that. More affected by higher body weight. Widely available over the counter in many countries and typically cheaper.

The choice often comes down to access and timing. If you can get to a clinic quickly and are open to an IUD, that is the gold standard. If you are buying a pill at a pharmacy, ellaOne is the stronger option, especially if more than 24 hours have passed or if your BMI is above 30. Levonorgestrel remains a good backup when it is the fastest option you can get your hands on, particularly within the first 24 hours.

The Cost Question

EllaOne typically costs more than levonorgestrel pills. In the UK, a cost-effectiveness analysis found that the incremental cost of preventing one additional unintended pregnancy with ellaOne compared to levonorgestrel was about £311. In sensitivity analyses spanning different time frames and assumptions, that figure ranged from £183 to £500, all of which fell well below the estimated cost of an unintended pregnancy (roughly £948) or an induced abortion (about £672).¹⁰ A separate French study looking at minors reached the same broad conclusion: the extra cost of ellaOne was justified by the additional pregnancies it prevented, generating savings for health insurers overall.¹¹

From an individual standpoint, whether the higher price tag matters depends on your situation. In countries where emergency contraception is covered by national health services or insurance, the cost difference is irrelevant to you. Where you are paying out of pocket, the extra expense buys you a wider effective window and somewhat better performance at higher body weights. If you are within 24 hours of unprotected sex and have normal body weight, the practical effectiveness gap between the two pills narrows, and levonorgestrel may be a perfectly reasonable choice if cost is a concern.

The Endometrial Effects Debate

Scientists have gone back and forth over whether ellaOne does anything beyond delaying ovulation. Some research has investigated whether the drug also affects the uterine lining in ways that could make it less hospitable to a fertilized egg. One study found that when ulipristal acetate was given around the time of ovulation, it altered gene expression in the endometrium in ways associated with reduced receptivity, potentially explaining why the drug’s pregnancy-prevention rates stay relatively high even on days when its ovulation-blocking ability has faded.¹² Another study examining the same question found that UPA affected the molecular machinery involved in decidualization, the process by which the uterine lining transforms to support a potential pregnancy.¹³

This is where the evidence gets complicated. The observation that ellaOne changes endometrial gene expression does not by itself prove that those changes matter in practice. The study that directly tested whether ellaOne prevents pregnancy after ovulation found that it does not work better than chance in that scenario.⁴ So the endometrial effects seen in lab analyses may not translate into a clinically meaningful anti-implantation effect. This question matters to some people for ethical or religious reasons, and the honest answer is that the science is not fully settled. What the clinical data does show clearly is that the drug’s primary effectiveness comes from preventing or delaying ovulation, and it does not end an existing pregnancy.

Vomiting, Drug Interactions, and Other Practical Pitfalls

Beyond the biological factors, a few practical issues can undermine ellaOne’s effectiveness. Vomiting within three hours of swallowing the tablet is the most obvious one. If this happens, you should contact a pharmacist or healthcare provider about taking a replacement dose. Anti-nausea medication taken beforehand can reduce this risk if you know you are sensitive.

Certain medications can also reduce ellaOne’s effectiveness. Drugs that induce liver enzymes, which speed up how quickly your body metabolizes medications, are the main culprits. These include some anti-seizure drugs (like carbamazepine and phenytoin), the antibiotic rifampicin (used for tuberculosis), and St. John’s Wort, the herbal supplement sometimes taken for mood. If you are taking any of these, ellaOne may be broken down too quickly to reach effective levels in your body. A copper IUD would be the preferred emergency contraceptive in that situation.

Antacids and proton pump inhibitors that raise stomach pH could theoretically affect absorption, though this has not been shown to be a clinically significant problem. If you are on regular medications and find yourself needing emergency contraception, mentioning those medications to the pharmacist is always worth the extra minute.

When a Copper IUD Is the Better Call

For certain people, ellaOne is not the best available option even when it is accessible. If any of the following apply, a copper IUD is worth pursuing:

BMI above 30: The pill’s effectiveness drops meaningfully at higher body weights. A copper IUD works regardless of weight.
Close to ovulation or unsure of cycle timing: If you suspect ovulation is imminent or has already occurred, the copper IUD provides protection that does not depend on blocking ovulation.
Enzyme-inducing medications: If you take drugs that speed up liver metabolism, the pill may not reach effective blood levels.
Wanting ongoing contraception: A copper IUD inserted for emergency purposes stays in place and provides highly effective contraception for years afterward, making it a two-in-one solution.

The barrier, of course, is access. Getting a copper IUD requires a healthcare provider trained in insertion, often on short notice, and that is not feasible for everyone. In places where walk-in sexual health clinics operate, same-day insertion is sometimes possible. Where it is not, ellaOne remains the strongest pill-based option and is vastly better than doing nothing. The evidence consistently places it ahead of levonorgestrel for most scenarios, and the cost analyses from multiple countries confirm that its higher price is offset by the additional pregnancies it prevents.¹⁰