ellaOne prevents roughly 6 out of 10 pregnancies that would otherwise occur after unprotected sex, with a failure rate between 0.9% and 2.1% in clinical trials. That makes it the most effective emergency contraceptive pill available. In a large clinical trial of over 1,500 women, the observed pregnancy rate was 1.9%, compared to an expected rate of 5.7% without any emergency contraception.
How the Numbers Break Down
The headline figure, a 2.1% pregnancy rate in the primary study population, can be misleading without context. That number doesn’t mean ellaOne fails one in fifty times. It means that out of every 100 women who took ellaOne after unprotected sex, about two became pregnant. Without the pill, roughly five or six of those women would have become pregnant. So ellaOne prevented about 62% of expected pregnancies overall.
The reason this number isn’t higher is that most women who have unprotected sex during their cycle won’t get pregnant anyway, regardless of emergency contraception. The pill only helps during the fertile window, and many women who take it weren’t at high risk to begin with. Among those who were genuinely at risk of pregnancy, the protective effect is substantial.
Effectiveness Over the 5-Day Window
ellaOne is licensed for use up to 120 hours (5 days) after unprotected sex, and its effectiveness holds remarkably steady across that window. In clinical trials, the pregnancy rates were 2.3% when taken between 48 and 72 hours, 2.1% between 72 and 96 hours, and actually dropped to 1.3% between 96 and 120 hours. No significant decline in efficacy was seen over time.
This is a major practical advantage. Levonorgestrel (the active ingredient in pills like Plan B and Levonelle) loses effectiveness as days pass and may be ineffective after 96 hours. ellaOne maintains its potency because of how it works at a biological level.
Why It Works Better Than Levonorgestrel
Both types of emergency contraceptive pill work by delaying ovulation, but ellaOne can do this at a later stage in the process. Your body releases an egg after a surge in luteinizing hormone (LH). Levonorgestrel can only block ovulation before that surge starts. ellaOne can still delay ovulation even after the surge has begun, during the 30 to 36 hours before it reaches its peak. Once the surge peaks, ellaOne’s ability to block ovulation drops sharply, but that extra window makes a real difference.
A meta-analysis comparing the two pills directly found that ellaOne reduced the odds of pregnancy by 42% more than levonorgestrel in the first 72 hours, and by 65% more in the first 24 hours. The advantage is greatest early on, but ellaOne also works in time frames where levonorgestrel has largely stopped being useful.
Body Weight and BMI
Body weight affects how well emergency contraception works, and this matters more for levonorgestrel than for ellaOne. Research has shown that levonorgestrel may be less effective in women over 165 pounds and possibly ineffective above 176 pounds. Obese women taking levonorgestrel had a fourfold greater risk of pregnancy compared to women of normal weight.
ellaOne holds up better at higher body weights. For women with a BMI over 30, the risk of unintended pregnancy with ellaOne is around 2.5%, compared to nearly 6% with levonorgestrel. If you weigh more than about 70 kg (154 pounds), ellaOne is the stronger choice among pill options. A copper IUD remains the most effective emergency contraception at any weight.
When ellaOne Won’t Work
ellaOne only works before ovulation. If you’ve already ovulated, the pill cannot prevent pregnancy because there’s already an egg available to be fertilized. The challenge is that most people don’t know exactly when they ovulate. If you’re unsure where you are in your cycle, taking ellaOne still makes sense because there’s a reasonable chance ovulation hasn’t happened yet.
Vomiting shortly after taking the pill can also reduce effectiveness. If you vomit within three hours of swallowing ellaOne, you need another dose because your body may not have absorbed enough of the medication.
Restarting Hormonal Contraception Afterward
If you use hormonal birth control (the pill, patch, ring, implant, or injection), there’s an important timing detail after taking ellaOne. You need to wait at least 5 full days before starting or restarting any contraception that contains progestin. Starting progestin-based contraception sooner can interfere with how ellaOne works, potentially making both less effective. During that waiting period, use condoms or avoid sex.
This waiting requirement doesn’t apply to levonorgestrel-based emergency contraception, which is one practical trade-off to be aware of when choosing between the two options.