The most effective emergency contraceptive is the copper IUD, which prevented 100% of expected pregnancies in clinical studies when placed within five days of unprotected sex. If you’re comparing pills specifically, ulipristal acetate (sold as ella) outperforms levonorgestrel (sold as Plan B One-Step and its generics), especially after the first 72 hours. Which option is “best” for you depends on timing, cost, access, and your body.
How Emergency Contraception Works
All forms of emergency contraception work primarily by preventing or delaying the release of an egg from your ovaries. If ovulation doesn’t happen, sperm has nothing to fertilize. This is the same basic mechanism as regular birth control pills, just delivered in a higher dose or through a different method. Emergency contraception does not end an existing pregnancy.
The Copper IUD: Most Effective Option
A copper IUD placed within five days of unprotected sex is the gold standard for emergency contraception. In a study of 318 participants, not a single pregnancy occurred, even though researchers estimated 15 to 23 pregnancies would have been expected without it. That’s a 100% prevention rate in the study group.
The copper IUD has two major advantages beyond raw effectiveness. First, its window is more flexible: when your ovulation day can be estimated, it can be placed even beyond five days after sex, as long as it’s not more than five days after ovulation. Second, once it’s in, it doubles as long-term birth control for up to 10 years. The downsides are that it requires a clinic visit, a provider to insert it, and an upfront cost that’s higher than a pill, though insurance often covers it.
Ella vs. Plan B: Comparing the Two Pills
Within the first three days after unprotected sex, ella (ulipristal acetate) and Plan B (levonorgestrel) have similar effectiveness. The real difference shows up in days three through five. CDC guidelines note that ella has been observed to be more effective than Plan B during that later window, making it the better pill option when more time has passed.
Plan B’s effectiveness drops sharply with each passing day. Taken within 24 hours, it’s about 94% effective. By 72 hours, that number falls to roughly 58%. Plan B is not recommended beyond 72 hours, though the five-day window technically applies to all emergency contraceptive pills.
There’s also a third pill option: the combined estrogen and progestin regimen (sometimes called the Yuzpe method). It’s less effective than either ella or Plan B and causes more nausea and vomiting, so it’s generally only used when nothing else is available.
Timing Matters More Than Brand
Regardless of which pill you choose, taking it sooner dramatically improves your odds. Every hour counts. If you have Plan B in your medicine cabinet right now and unprotected sex happened in the last 24 hours, that convenience alone may make it the best practical choice. A theoretically superior option you can’t get for two days is worse than a good option you can take immediately.
That said, if you’re within the first day or two and can get a prescription for ella or an appointment for a copper IUD, those options give you a meaningful edge, particularly if you’re approaching the 72-hour mark.
Does Body Weight Affect Effectiveness?
This is a genuinely unsettled question. Some studies have found that Plan B becomes less effective at higher body weights. One meta-analysis found pregnancy rates of about 1% for women with a normal BMI, 2.4% for those with a BMI of 25 to 30, and 5.2% for those with a BMI over 30. But a separate analysis of three large studies found no such trend at all.
Ella shows a similar pattern of uncertainty. Limited data suggest a possible decline in effectiveness at higher weights, but the evidence isn’t strong enough to draw firm conclusions. The European Medicines Agency reviewed all available data and concluded that both pills can be used by women of all weights because the benefits still outweigh the risks. Still, if weight is a concern for you, the copper IUD’s near-perfect effectiveness is unaffected by body size.
Cost and How to Get Each Option
Plan B and its generics are available over the counter with no age restriction and no prescription. Brand-name Plan B One-Step typically costs $40 to $50. Generic versions (sold under names like Take Action, My Way, Aftera, and EContra) can cost as little as $11 to $19, depending on the retailer.
Ella requires a prescription, which adds a step. Out of pocket, it runs about $44 to $45. Some telehealth platforms offer a consultation and prescription for a small fee, and insurance often covers the cost entirely. If you think you might need ella in the future, getting a prescription in advance saves critical time.
The copper IUD is the most expensive upfront if you’re paying out of pocket, but most insurance plans cover it fully under the Affordable Care Act’s contraceptive mandate. Since it also serves as long-term contraception, the per-year cost is actually very low.
What to Expect After Taking It
The most common side effects of both Plan B and ella are nausea, headache, fatigue, and some abdominal discomfort. Your next period may come a few days early or late, and it might be heavier or lighter than usual. These effects are temporary and typically resolve within a day or two. If your period is more than a week late, take a pregnancy test.
One important interaction to know: if you take ella, you should not use hormonal birth control (like the pill, patch, or ring) for at least five days afterward, because the hormones can interfere with how ella works. Plan B doesn’t have this restriction, so you can resume or start hormonal birth control right away.
Quick Comparison
- Copper IUD: Most effective overall (nearly 100%), works up to 5 days, requires a clinic visit, doubles as long-term birth control
- Ella (ulipristal acetate): Most effective pill, maintains effectiveness through day 5, requires a prescription, costs $44 to $45 without insurance
- Plan B (levonorgestrel): Most accessible, available over the counter at any age, best within 24 hours (94% effective), drops to about 58% by 72 hours, generics available for under $20
If you want the single most effective emergency contraceptive regardless of logistics, it’s the copper IUD. If you want the best pill and can get a prescription quickly, it’s ella. If you need something right now with no barriers, a generic levonorgestrel pill from your nearest pharmacy is a solid, well-studied option that works best the sooner you take it.