Plan B becomes significantly less effective if you weigh over 165 pounds. A meta-analysis of clinical data found that women with obesity who took Plan B had three to four times the risk of pregnancy compared to women at a normal weight. The European manufacturer of levonorgestrel emergency contraception changed its labeling to warn that the pill is less effective at 165 pounds and potentially ineffective above 176 pounds. That said, “less effective” is not the same as “zero effect,” and better alternatives exist.
Why Weight Reduces Plan B’s Effectiveness
Plan B works by delivering a single dose of a synthetic hormone that prevents or delays ovulation. When your body has more tissue for that hormone to spread through, the concentration in your blood drops. In pharmacology terms, the drug’s “volume of distribution” roughly triples in women with obesity compared to normal-weight women. That means the same 1.5 mg dose gets diluted across a much larger space.
The issue goes beyond simple dilution. Your body carries this hormone partly bound to a protein in the blood called SHBG. Women at higher weights tend to have lower levels of SHBG, which means more of the hormone floats freely and can drift out of the bloodstream into fat tissue. Fat tissue may also break down the hormone on its own, further reducing how much is available to actually suppress ovulation. The net result: the total amount of usable hormone circulating in the blood is significantly lower in women with obesity, and that’s the hormone that needs to reach a high enough level to stop an egg from being released.
Doubling the Dose Doesn’t Fix the Problem
A logical thought is to just take two Plan B pills instead of one. Researchers tested exactly this in a randomized controlled trial involving women who weighed at least 176 pounds with a BMI over 30. The double dose (3.0 mg instead of 1.5 mg) did not significantly improve outcomes. Among women who took the standard dose, about 51% had no ovulation within five days. Among those who took the double dose, about 69% didn’t ovulate, but the difference was not statistically meaningful. The timing of ovulation in women who did ovulate was identical in both groups. Taking two pills is not a reliable workaround.
Alternatives That Work at Any Weight
The American College of Obstetricians and Gynecologists is clear on this: regardless of body weight, the copper IUD is the most effective form of emergency contraception. It can be inserted up to five days after unprotected sex and reduces the risk of pregnancy by more than 99%. It also doubles as long-term birth control for up to 10 years. The downside is that you need a provider to place it, which may require a same-day or next-day appointment.
The second-best option is ella (ulipristal acetate), a prescription emergency contraceptive pill that works up to five days (120 hours) after unprotected sex. It loses effectiveness at a higher weight threshold than Plan B does, though it may still become somewhat less effective for women with a BMI of 30 or above. For someone weighing between 165 and 195 pounds, ella is a considerably better pill option than Plan B. You’ll need a prescription or a telehealth visit to get it, but many online services can provide one quickly.
Should You Still Take Plan B Over Nothing?
Yes. Reproductive health experts have pushed back against the idea that Plan B is completely useless above 165 pounds. The data shows reduced effectiveness, not zero effectiveness. If a copper IUD or ella aren’t accessible to you in the moment, taking Plan B is still better than doing nothing. The American College of Obstetricians and Gynecologists specifically states that women should not be refused or discouraged from using levonorgestrel emergency contraception because of their weight.
The 165-pound threshold is also not a hard cutoff where the pill suddenly stops working. It’s a point on a curve where effectiveness drops enough to matter clinically. A woman weighing 170 pounds will likely get more benefit from Plan B than a woman weighing 210 pounds, but neither should assume the pill is worthless. The key distinction is that if you have the time and access to get ella or a copper IUD instead, those options give you meaningfully better protection.
What the U.S. Label Currently Says
As of now, the U.S. label for Plan B does not include a weight-based warning. The FDA reviewed the data after European regulators added their warnings in 2013 but has not required a labeling change for the American market. Health Canada is the only regulator that explicitly recommends women over 165 pounds seek alternative emergency contraception. The original clinical trials that led to Plan B’s approval in the U.S. did not include a weight-related assessment of efficacy, which means the gap in labeling reflects a gap in how the drug was studied, not evidence that it works fine at higher weights.
The practical takeaway: if you weigh over 165 pounds and need emergency contraception, Plan B is your least effective pill option. Ella is better, and a copper IUD is the most reliable choice available. If Plan B is all you can access right now, take it, but look into ella or an IUD placement as a next step if you’re still within the five-day window.