The most effective emergency contraceptive is an IUD, not a pill. In clinical trials, IUDs used as emergency contraception had a pregnancy rate of just 0.3%, compared to 1.4–2.6% for oral options. Among pills specifically, a prescription option called ella outperforms the familiar Plan B One-Step. But the “best” choice depends on your weight, how much time has passed, and what you can access right now.
IUDs: The Most Effective Option
If you can get to a provider within five days of unprotected sex, an IUD is the gold standard. A study funded by the National Institutes of Health found that neither the copper IUD nor the hormonal IUD produced more than one pregnancy among over 600 participants. That 0.3% pregnancy rate is roughly five to eight times better than any pill.
The copper IUD (ParaGard) has been used as emergency contraception for decades, but research now shows a hormonal IUD works just as well for this purpose, with less discomfort during insertion. Both types double as long-term birth control afterward, lasting anywhere from five to twelve years depending on the device. The tradeoff is that you need a clinic appointment, which isn’t always realistic within the window.
Ella vs. Plan B: How the Pills Compare
Among oral emergency contraceptives, ella (ulipristal acetate) is more effective than Plan B (levonorgestrel). A meta-analysis published in The Lancet found that within 72 hours of intercourse, ella had a 1.4% pregnancy rate compared to 2.2% for Plan B. That difference was statistically significant.
The gap between them widens the longer you wait. Plan B works best in the first 24 hours and loses effectiveness steadily over three days. Ella maintains its effectiveness across the full five-day (120-hour) window, which is a major practical advantage if a day or two has already passed. Both pills work by delaying or preventing ovulation. Neither one is effective if ovulation has already happened. Research confirms that when levonorgestrel is taken after ovulation, conception rates are essentially the same as with no contraception at all.
The catch: ella requires a prescription, while Plan B One-Step and its generic equivalents are available over the counter to anyone, at any age, with no ID required. Two-pill generic versions of levonorgestrel are sold from behind the pharmacy counter to people 17 and older. If speed matters more than anything, the pill you can get right now is better than the one you have to wait for.
Why Weight Changes the Equation
Body weight is one of the most important and least discussed factors in emergency contraception. Plan B’s effectiveness drops significantly as weight increases. Research shows it begins losing efficacy at around 155 pounds (70 kg) and appears to have essentially no effect at 176 pounds (80 kg) or above. People with a BMI of 30 or higher who took Plan B had more than four times the risk of pregnancy compared to those with a BMI under 25.
Ella holds up somewhat better at higher weights, though its effectiveness also declines with obesity. In one trial of women weighing 176 pounds or more, researchers found no significant difference in pregnancy rates between ella and a double dose of levonorgestrel, though the study was too small to draw firm conclusions.
For people over 155 pounds, the practical takeaway is clear: an IUD is the most reliable choice. If that’s not accessible, ella is a better option than Plan B. Health Canada is currently the only regulator that explicitly warns consumers about these weight thresholds on product labeling.
Timing Matters More Than Brand
Regardless of which option you choose, speed is everything. Plan B is most effective in the first 24 hours after unprotected sex and can be taken up to 72 hours afterward, though its effectiveness drops with each passing day. Ella works for up to 120 hours (five days) and maintains more consistent protection across that window. IUDs can also be placed within five days.
One timing detail worth knowing: if you vomit within three hours of taking either pill, the dose may not have been absorbed. In that case, you likely need to take another one.
Medications That Reduce Effectiveness
Certain drugs speed up how quickly your liver breaks down levonorgestrel, which can make Plan B less effective or even useless. The main culprits include some epilepsy medications (phenytoin, carbamazepine, barbiturates), the tuberculosis drug rifampicin, certain HIV treatments, the antifungal griseofulvin, and St. John’s wort. If you take any of these, doubling the standard levonorgestrel dose from one pill to two is sometimes recommended, though an IUD or ella may be a better route entirely.
Choosing the Right Option
- Best overall effectiveness: A copper or hormonal IUD placed within five days. Requires a provider visit but offers a 0.3% failure rate and becomes ongoing contraception.
- Best pill option: Ella, especially if more than 24 hours have passed or you weigh over 155 pounds. Requires a prescription.
- Most accessible option: Plan B One-Step or a generic levonorgestrel pill. Available without a prescription at any pharmacy. Most effective when taken within 24 hours by someone under 155 pounds.
If you’re under 155 pounds and it’s been less than a day, Plan B and ella will both work well. The further you get from that scenario, in time or weight, the more the advantage shifts toward ella or an IUD. The single best predictor of success with any method is how quickly you use it.