Plan B is an emergency contraceptive designed to prevent pregnancy after unprotected sex or contraceptive failure. Many people want to know what weight Plan B works for and how body size can affect its reliability.
This article outlines how weight and BMI influence effectiveness, timing options, and what to do if you are above a certain weight. Use the tables and sections below to find practical, evidence-based guidance.
| Weight Range | BMI Range | Effectiveness for Levonorgestrel | Recommended Action |
|---|---|---|---|
| Below 75 kg | Below 30 | Highly effective | Use Plan B as soon as possible |
| 75–90 kg | 30–35 | Reduced effectiveness | Consider ulipristal acetate or a copper IUD |
| Above 90 kg | Above 35 | Markedly reduced for levonorgestrel | Use ulipristal acetate or a copper IUD if eligible |
| Any weight | All ranges | Copper IUD highly effective | Opt for IUD for most reliable emergency contraception |
How Body Weight Affects Plan B
Pharmacokinetics in larger bodies
Larger body mass and higher body fat can change how levonorgestrel is absorbed and distributed. This can lower the peak hormone concentration and reduce the chance of preventing ovulation.
Impact on ovulation suppression
Studies suggest that Plan B is less likely to stop ovulation in people with higher weight. If ovulation is not delayed, the risk of pregnancy remains higher compared with lower weight individuals.
Timing Matters Alongside Weight
What to do after unprotected sex
Take Plan B as soon as you can, ideally within 72 hours. If you are above the weight threshold where levonorgestrel is less reliable, seek ulipristal acetate or a copper IUD instead.
Weight thresholds for levonorgestrel
Guidelines often indicate reduced effectiveness above 70 kg, with significant reduction above 75–80 kg. This makes ulipristal acetate or a copper IUD preferable options for many people in larger body sizes.
Emergency Contraception Options by Body Size
Ulipristal acetate considerations
Ulipristal acetate maintains better effectiveness up to 85–90 kg for many people. It requires a prescription and should be taken within 120 hours, but it is less impacted by weight than levonorgestrel.
Copper intrauterine device (IUD)
A copper IUD is the most reliable form of emergency contraception regardless of weight. Inserted within five days, it is over 99% effective and also provides ongoing contraception.
Key Takeaways on Weight and Emergency Contraception
- Effectiveness of levonorgestrel drops above 70–75 kg due to body weight and fat distribution.
- Ulipristal acetate works better at higher weights, but a copper IUD is most reliable for everyone.
- Take any emergency contraception as soon as possible after unprotected sex.
- Consult a healthcare provider to choose the best option based on your weight and situation.
- Consider ongoing contraception methods that fit your body and lifestyle to reduce future risk.
FAQ
Reader questions
Does Plan B work effectively for people over 90 kg?
Levonorgestrel is much less reliable above 90 kg. Ulipristal acetate or a copper IUD is recommended for higher effectiveness in this weight range.
Can I use Plan B if I weigh 80 kg?
You can, but it may be less effective. Ulipristal acetate or a copper IUD is preferred for better reliability if your weight is around 80 kg or higher.
Will my weight affect how soon Plan B works?
Weight can delay hormone absorption and reduce ovulation suppression, which may lower the chance of preventing pregnancy.
Is a copper IUD a better choice for me if I am heavier?
Yes, a copper IUD works independently of body weight and is the most reliable emergency contraception option for people of all sizes.