Morning-after pills do not have an official weight limit printed on their packaging, but clinical evidence shows they become significantly less effective as body weight increases. The most common type, sold as Plan B One-Step and its generics, starts losing effectiveness at around 165 pounds (a BMI of 26). A prescription alternative called ella holds up better at higher weights but also declines starting around 194 pounds (a BMI of 35). Neither pill is banned or contraindicated at any weight, but the protection they offer drops enough that other options are worth considering.
Plan B Effectiveness by Weight
Plan B and its generic versions contain a hormone called levonorgestrel. It works primarily by delaying ovulation, preventing an egg from being released so sperm have nothing to fertilize. The problem is that in people with higher body weight, blood levels of this hormone are substantially lower after taking the same dose. In one pharmacokinetic study, peak hormone levels in obese participants were about 35% lower than in normal-weight participants, and overall drug exposure was roughly 45% lower.
The reason is straightforward: levonorgestrel is a fat-soluble drug. In someone with more body tissue, the drug spreads across a much larger volume instead of staying concentrated in the bloodstream. Researchers measured the distribution volume at roughly 400 to 466 liters in obese participants compared to 162 liters in normal-weight participants. With more tissue absorbing the drug, less of it remains available to suppress ovulation.
In practical terms, one analysis found that for people with a BMI of 26 or higher, pregnancy rates after taking levonorgestrel-based emergency contraception were no different than if they had not used emergency contraception at all. That BMI of 26 corresponds to roughly 165 pounds for someone of average height. At a BMI over 30, the failure rate climbs to around 6%, compared to much lower rates in normal-weight users.
Does Doubling the Dose Help?
Some health organizations, including the UK’s Faculty of Sexual and Reproductive Healthcare, have suggested that people over 70 kg (about 154 pounds) consider taking a double dose of levonorgestrel (3 mg instead of 1.5 mg). The American Society for Emergency Contraception has made a similar suggestion for those with a BMI over 30. It sounds logical: if the drug is being diluted, just take more of it.
A randomized study tested this directly in people with a BMI over 30 who weighed at least 176 pounds. The results were clear: doubling the dose did not improve ovulation suppression. In the standard-dose group, about 51% of participants had no egg release within five days. In the double-dose group, that number was 69%, but the difference was not statistically significant. Among those who did ovulate, the timing was identical in both groups. The researchers concluded that simply doubling the dose is not an effective strategy for improving outcomes at higher weights.
How Ella Compares
Ella (ulipristal acetate) is a prescription-only emergency contraceptive that works through a different mechanism. It’s effective up to five days after unprotected sex, compared to Plan B’s three-day window, and it maintains better efficacy across a wider weight range. The American College of Obstetricians and Gynecologists notes that ulipristal acetate is more effective than levonorgestrel at all time points and across all weight categories.
That said, ella is not immune to weight effects. Its efficacy begins to decline at a BMI of 35, which corresponds to roughly 194 pounds for an average-height person. For people with a BMI over 30, the unintended pregnancy rate with ella is about 2.5%, compared to roughly 6% with levonorgestrel. That’s a meaningful difference, but it still represents reduced protection compared to what normal-weight users experience.
Ella requires a prescription in the United States, which can be a barrier in time-sensitive situations. Some telehealth services offer rapid prescriptions, and certain pharmacies can dispense it through collaborative practice agreements.
The Copper IUD as Emergency Contraception
The copper IUD is the most effective form of emergency contraception regardless of body weight. It can be inserted up to five days after unprotected intercourse and reduces the risk of pregnancy by more than 99%. Unlike pills, its mechanism is local rather than systemic: the copper creates an environment in the uterus that is toxic to sperm and prevents implantation. Body weight does not dilute this effect.
Beyond emergency use, the copper IUD continues working as ongoing contraception for up to 12 years. The downside is access. It requires a clinic visit and a provider who can insert it, which may not be realistic within the five-day window for everyone. Still, for people over 194 pounds who want the most reliable emergency protection, it is the option most consistently recommended by major medical organizations.
Choosing the Right Option by Weight
If you weigh under 165 pounds, Plan B and its generics work as intended and are the most accessible option, available over the counter without a prescription or age restriction.
Between 165 and 194 pounds, ella is the better pill-based choice. Plan B’s effectiveness in this range is significantly reduced, while ella still provides meaningful protection. If you cannot get a prescription for ella quickly enough, taking Plan B is still better than taking nothing, but expectations should be adjusted.
Above 194 pounds, neither pill option offers strong protection. Ella’s effectiveness is declining at this point, and levonorgestrel-based pills are largely ineffective. The copper IUD is the recommended option in this weight range. If IUD insertion is not possible, ella remains the better of the two pill options, but its reliability is limited.
These thresholds are approximate and based on BMI, which means the exact weight cutoffs shift depending on your height. A person who is 5’2″ will hit a BMI of 26 at a lower weight than someone who is 5’8″. The numbers cited (165 and 194 pounds) are based on average female height in studies and should be treated as general guideposts rather than hard lines.