While Plan B, the morning-after pill, is generally very effective in preventing a pregnancy, some studies suggest that the active ingredient, Levonorgestrel (LNG), may not work as well for people who weigh more than 155 pounds or have a BMI above 25.
More than one in four women in the U.S. have a BMI between 25 and 29.9, while two in five have a BMI above 30, according to the 2018 National Health and Nutrition Examination Survey (NHANES), conducted by the National Center for Health Statistics.
In other words, the most popular form of emergency contraception may not actually work well for a substantial portion of the population.
While data are still somewhat limited, a study found that women with obesity experienced a four-fold increased risk of pregnancy after LNG use compared with those with a BMI under 30.
And one meta-analysis found that women with a BMI over 25 have a greater risk of getting pregnant after taking LNG, leading European emergency contraceptive company NorLevo to update its product labeling to warn consumers about reduced efficacy with higher BMIs.
Experts say the reduced effectiveness likely has to do with the concentration of the medication in the body.
“Medications must reach a certain concentration in the body to be effective,” Cecily D. Havert, MD, a physician specializing in issues that affect women and the LGBTQ+ community, told Verywell. “In people with a BMI [above] 30, the levonorgestrel medication does not reach a level in the body that can inhibit ovulation and therefore does not prevent pregnancy as well as in someone with a lower BMI.”
The Alternatives
Comprehensive, large-scale research on the efficacy of morning-after pills is lacking, which is why most over-the-counter brands don’t spell this out on their packaging. Still, existing data suggest that people with a BMI above 25 should explore alternative emergency contraception options.
Another oral pill, called ella, contains the drug ulipristal acetate (UPA), which tends to be more effective for people with higher BMIs, Havert said. However, it requires a prescription from a healthcare provider and typically costs $50 or more without insurance. According to Planned Parenthood, this pill also loses some efficacy if the person taking it weighs 195 pounds or more.
Another highly effective alternative is an intrauterine device (IUD), Havert suggested. If it’s inserted within five days of unprotected intercourse or during the cycle when a pregnancy test is negative, then the risk of pregnancy is between 0.1 and 0.3%, depending on the type of IUD used, regardless of one’s BMI.
But these alternatives aren’t without barriers.
“When choosing another EC medication such as UPA, barriers include the need to have healthcare provider’s prescription, which would frequently involve a medical appointment,” she said. “The insertion of an IUD is also a process that may take time to arrange, and the device is not always covered well by insurance.”
Though Plan B can also be expensive (up to $40 for a pill), it may be the only accessible option in a time crunch.
That’s why Tanja Premru-Sršen, MD, PhD, a specialist in fetal-maternal medicine and ultrasound diagnostic in pregnancy and a chief medical advisor at Bellabeat, told Verywell that time is the most crucial factor to consider when discussing emergency contraception options.
She said patients should be informed about the potential for reduced efficacy, but the morning-after pill should still be an option for anyone who needs it.
“Access to emergency contraceptive pill with levonorgestrel should still be promoted to women who need them and not be restricted in any weight or BMI category,” she said. “It is important to note to use Plan B as directed no matter what your weight or BMI is.”
