Yes: levonorgestrel emergency contraception may not work if you weigh more than 165 pounds.
The likely mechanism is dose dilution: a fixed 1.5 mg dose yields lower blood levels in a larger body, and delaying ovulation requires crossing a hormone threshold. Ella (ulipristal) performs better at higher weights but fades too. The copper IUD is immune to the problem because it works inside the uterus, not through the bloodstream; it is the most effective emergency contraception at any weight (over 99% within 5 days of unprotected sex) and doubles as up to 10 years of ongoing birth control. So the weight-based triage is: over 165 pounds, aim for a copper IUD first or ella second. If Plan B is all you can get today, take it anyway, since some protection beats none, and take a pregnancy test if your period has not come within 3 weeks.
What to do
Common questions
Does weight affect Plan B?
Yes, body weight appears to reduce Plan B's effectiveness. Planned Parenthood's guidance, reflecting the research on levonorgestrel EC, is that these pills may not work if you weigh more than 165 pounds. The leading explanation is dilution: the same 1.5 mg dose produces lower blood hormone levels in a larger body, and levonorgestrel's job of delaying ovulation depends on hitting a threshold level. Ulipristal (ella) holds up somewhat better at higher weights but also loses effectiveness as weight climbs. The option that weight does not touch is the copper IUD: it works locally in the uterus, not through the bloodstream, so it is equally effective at any weight, remains the most effective emergency contraception overall (over 99% when placed within 5 days), and then continues as regular birth control for up to 10 years. Practical advice: if you weigh over 165 pounds and need EC, the copper IUD is the strongest choice and ella is the better pill. If Plan B is genuinely the only thing available, taking it is still better than taking nothing, but follow up with a pregnancy test if your period does not arrive within 3 weeks.