Plan B does not have an official weight limit on its label, but research shows it starts losing effectiveness at around 165 pounds (75 kg) and may provide little to no benefit at 176 pounds (80 kg) and above. The FDA-approved labeling for Plan B One-Step does not mention a weight or BMI cutoff, so the pill is technically available to anyone. But the clinical data tells a different story, and understanding where your weight falls can help you choose the most effective option.
Where Effectiveness Starts to Drop
The clearest evidence comes from two randomized trials analyzed by the American Academy of Family Physicians. For women with a BMI over 30 (roughly 175 pounds for someone 5’4″), the pregnancy rate after taking Plan B was 5.8%. The pregnancy rate for women who took no emergency contraception at all was 5.6%. That difference is essentially zero, meaning Plan B provided no measurable protection at that weight.
The decline begins before that point. Women with a BMI over 25 (around 145 to 150 pounds for the same height) already showed a higher risk of pregnancy compared to lighter women. And at a BMI above 26, pregnancy rates were statistically the same as using nothing. Clinical trial data suggests Plan B hits a ceiling of effectiveness around 154 pounds (70 kg) and loses efficacy entirely around 176 pounds (80 kg).
Why Weight Affects a Hormone-Based Pill
Plan B works by delivering a large dose of a synthetic hormone called levonorgestrel, which delays or prevents ovulation. In women with more body mass, blood levels of this hormone run about 30% lower than in lighter women given the same dose. The result is that the drug may not reach the concentration needed to reliably block ovulation.
The reason isn’t fully settled, but the leading explanation involves how the body processes fat-soluble compounds. Levonorgestrel binds to proteins in the blood, and differences in metabolism rates among heavier individuals mean the drug clears faster or distributes into a larger volume of tissue. That lower concentration can allow ovulation to happen anyway, which is exactly what the pill is supposed to prevent. It also takes longer to reach peak levels, which matters when timing is everything.
Whether Doubling the Dose Helps
Researchers at Oregon Health & Science University tested whether taking two Plan B pills (3 mg of levonorgestrel instead of the standard 1.5 mg) could restore effectiveness in women with obesity. The logic was straightforward: if blood levels are 30% lower, a higher dose might compensate. A randomized controlled trial published in Obstetrics & Gynecology in 2022 evaluated this approach, comparing follicle rupture (a marker of ovulation) in women taking the standard versus double dose.
This is not a strategy you should assume works on your own. The study was designed to test the concept, and doubling a dose without clinical guidance carries its own risks. No major medical organization currently recommends taking two pills as standard practice for heavier individuals.
Options That Work Regardless of Weight
The most effective form of emergency contraception at any weight is the copper IUD. Inserted by a provider within five days of unprotected sex, it works by creating an environment that prevents fertilization. Multiple studies confirm that the copper IUD’s performance is completely unaffected by BMI. Even at a BMI above 40, pregnancy rates did not increase (though expulsion rates were slightly higher in that range).
The second-best option is a prescription pill called ella (ulipristal acetate). The American College of Obstetricians and Gynecologists recommends it over Plan B when possible, noting it is more effective at all time points up to five days after unprotected sex and across all weight categories. It does lose some effectiveness in heavier women, but at a higher weight threshold than Plan B. It requires a prescription, which can be obtained through telehealth services or an urgent care visit.
ACOG’s guidance is clear on one point: women who are overweight or obese should not be refused Plan B or discouraged from using it based on weight alone. If it’s the only option available, taking it is still reasonable. But if you have the ability to access ella or a copper IUD, those are stronger choices for anyone over about 150 to 165 pounds.
How Timing and Weight Interact
Plan B is most effective when taken as soon as possible after unprotected sex, with a maximum window of 72 hours. For lighter women, effectiveness declines gradually across that window. For heavier women, the margin is even thinner. Because the drug already reaches lower blood concentrations and takes longer to hit peak levels, any delay compounds the problem. If you’re near or above the weight threshold, acting within the first 12 to 24 hours gives you the best chance of the pill doing anything at all.
Ella has a longer effective window of up to 120 hours (five days) and maintains its effectiveness more consistently across that period, which is another reason providers recommend it for women concerned about weight-related efficacy loss. The copper IUD shares that five-day window and is the only method whose effectiveness does not change with either body weight or the number of hours elapsed.