You might not know this, since it doesn’t appear anywhere on the medication itself, but the morning-after pill doesn’t necessarily work for women over 176 pounds.

This shouldn’t be niche information, but it is: Though the weight limits for emergency contraceptives have been publicly disputed since at least 2011, you might not be aware of the debate. Until one of us happened upon a tweet about it yesterday, a bunch of us at Fusion didn’t know, either.

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The average weight of an American woman in 2013, according to the CDC, was 166 pounds. For black women between the ages of 20 and 39, it was 186. Which means a “normal” dose of Plan B could be near-useless for a huge swath of American women.

The emergency contraceptive levonorgestrel—aka Plan B—was available in the United States for nearly 15 years before the idea that it wasn’t effective for women above a certain weight started getting attention.

In 2011, a study in Contraceptive journal recommended that women with a BMI of above 25 shouldn’t rely on emergency contraception. A few years later a French manufacturer of a Plan B clone told Mother Jones it was changing its packaging to warn women that the drug started to lose effectiveness if they weighed more than 165 pounds, and was essentially useless if they were more than 176. (That manufacturer later removed the warning label, after the European Medicines Agency claimed available data was too limited to prove anything—an omission which lends itself to our current predicament.)

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In the years since, researchers and manufacturers have volleyed over the point. Plan B claims there are no weight limits, but some researchers insist women above a certain BMI should take two doses—bringing the cost of an emergency contraceptive to around $100. And statistically, the less money you make, the more likely you are to be over 176 pounds.

And you’d only know to get that second $50 dose if you’d googled around. Here at the Gizmodo Media Group office, even among women who make it their business to know these kinds of things, only 9 out of 56 women polled had any idea this was even a question.

Bizarre that a product so explicitly targeted to the needs of the “average” woman would appear to have been tested and approved using a sample group that was smaller and, in all likelihood, whiter that the general population. (In ‘99, the year the FDA approved Plan B, the average weight was still around 160.) Institutional blind spots like this happen all the time: It’s a little like how female crash dummies weren’t used in car testing until 2012, or standardized kitchen counter heights are absolutely perfect for women who are 5' 7".

Perhaps one of the reasons conclusive data on this issue doesn’t exists is that it’s risky to tell heavier women to buy an extra dose: Theoretically, if you made women over a certain weight shell out an extra $50, it could be grounds for a class action suit.