Does weight affect Plan B?
Last updated September 3, 2026.
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
- Know the number: over 165 pounds, levonorgestrel EC may not work.
- Ask for the copper IUD: weight does not affect it, and it is the most effective EC.
- Ella is the better pill: at higher weights it outperforms Plan B, but it needs a prescription.
- Plan B beats nothing: if it is all you can get, take it fast and follow up.
- Test at 3 weeks: no period by then means take a pregnancy test.
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