You may think your emergency contraceptive is 95 percent effective, but for women who surpass its "weight limit" it may not be so reliable.

By Moira Lawler
Updated: September 16, 2016
Corbis Images

Many women turn to the morning-after pill to prevent pregnancy when they forgo protection in the heat of the moment-or if another form of contraception fails (like a broken condom). And for the most part, the morning-after pill is a safe and reliable method. But there's a catch: It may not be effective if you're overweight, according to a new study published in the journal Contraception.

For the study, researchers gave a group of 10 women with normal and obese BMIs 1.5 milligrams of levonorgestrel-based emergency contraception. Afterward, researchers measured the concentration of the hormone in the women's bloodstreams. They found the concentration significantly lower (meaning it was less effective) among the obese participants than among those in the normal BMI range. So the researchers gave the obese group a second round, this time at double the dose. That kicked the concentration levels up to what normal-weight participants had after just a single dose. Pretty big difference.

But that doesn't mean heavier women should just double their dose of EC and call it a day. There haven't been enough studies done yet to prove whether that's a sustainable preventative method, or if it could potentially stop ovulation. (Related: How Bad Is It to Take Plan B As Regular Birth Control?)

This news resurfaces concerns about emergency contraception's effectiveness, given that in 2014 a European brand called Norlevo started to include a warning on its label that the pill may not be effective for women over 165 pounds (the Average American woman weighs 166 pounds, according to the CDC). And for women over 175 pounds? It didn't work at all. That matters to those of us in the U.S. because Norlevo is chemically identical to the one- and two-pill versions of the Plan B we get stateside. According to the Centers for Disease Control and Prevention, the average woman in the U.S. weighs 166 pounds. So a lot of women could be affected.

Bottom line: Being overweight can keep levonorgestrel-based EC from effectively preventing pregnancy. And while researchers found success in doubling the dosage among overweight patients, they say more research is needed before they can absolutely recommend that approach. In the meantime, women with a BMI greater than 25 should opt for the EC Ella, which is thought to be more effective for women with higher body weights, or a copper IUD, which can be inserted up to five days after sex, according to another study published in Contraception.



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