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The thought of accidental pregnancy is, to put it mildly, a bummer. But one factor that women who face this situation might not consider is how their weight might effect their options.

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Recently, a small study published in the journal Contraception revealed a potentially critical finding when it comes to emergency contraception: Obese women might require up to twice the dose of more lithe women for the drug to effectively prevent pregnancy.

Much like your favorite pair of jeans, one dose of emergency contraception might not fit all.

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To understand the research, it's helpful to first understand the mechanism behind emergency contraception, aka the morning-after pill, aka Plan B.

Contrary to what you might have heard, the morning-after pill isn’t the abortion pill—it doesn’t terminate a pregnancy. What the morning-after pill does is block ovulation, thus preventing the chance that an egg will be released and find its way to willing sperm, stopping the possibility of conception. It basically works like a mega dose of birth control pills, tricking the body into not following through with the hormonal process that leads to an egg being released, explains Jeffrey Jensen, one of the study’s authors and an OBGYN at Oregon Health and Science University.

When you take regular oral birth control pills, the drug releases a combination of progestin and estrogen in small but steady levels to create a hormonal baseline in the body. A daily dose may create a spike when first taken—in the same way that eating a candy bar creates a sugar spike—but ultimately, a woman's hormones level out into the kind of sustained, therapeutic baseline needed to stop the body from ovulating.

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The morning-after pill is about creating a similar end-result but with rapid onset and fast action. You need a high level—immediately—of the synthetic hormone progestin for the body to react in the same way as it would to the small, steady dose of oral contraception.

So why does weight matter? Based on previous studies, obese women appear to experience a delay in reaching the peak level needed for the drug to be effective. The researchers surmise that, when a woman has a higher body mass index, or BMI, she simply needs more of the drug in her system for her body to register it and for it to take effect.

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So Jensen and the research team at OHSU decided to explore the simple strategy of doubling the current over-the-counter dosage of the morning-after pill in women whose body mass index indicated obesity, to see if it impacted the drug’s efficacy at preventing pregnancy.

Turned out, in this very early experiment, the strategy worked. The study looked at 10 women, five with “normal” level BMIs and five with BMIs that indicate obesity. After taking the current recommended dose of the drug, the women in the obese group had their blood tested—and it registered insufficient levels of the hormones needed to prevent pregnancy. When their dose was doubled, however, the obese women’s hormone levels matched those of the non-obese women who had taken just one dose. (Just to be clear: None of these women were at risk for pregnancy. The researchers were simply looking at the hormone levels present in their blood.)

While the research they’ve conducted is preliminary—and the number of participants in the study was tiny—Jensen believes healthcare providers should start taking the finding into account. “These hormones are extremely safe,” Jensen explains. “You can’t overdose on them.” So while the research is still nascent, “we should still feel comfortable that this is not a risky strategy.”

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That said, the sample size in the study was too small to notice any trends involving unpleasant side effects such as increased nausea or headaches.

According to Jensen, the biggest downside of doubling up on Plan B would be cost—double the pills, double the price tag. The Plan B brand of emergency contraception typically costs anywhere from $40 to $50 a dose, while Next Choice, Take Action, and My Way typically are less expensive, somewhere around $35 to $45 a dose. Some insurance plans cover this cost, but might require a prescription to do so; free or low-cost emergency contraception is often also available through Planned Parenthood or public health centers.

The cost factor should not be overlooked, however, especially given that a 2015 study in the Journal of Pediatrics found that obese teenagers are less likely to use prescription contraception than their “normal” weight peers. Research suggests this population may already feel self-conscious and concerned about stigma associated with their weight; added cost would present yet another barrier.

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Jensen notes that Plan B is effective about 80% of the time when given to women before ovulation. While some women will still ovulate and become pregnant, others will have ovulation successfully blocked. Very often, this just comes down to the old adage that timing is everything—which is why it’s so important to take emergency contraception as soon as you realize you might need it. If you don’t take it in the right window of your body’s hormonal cycle, it unfortunately just can’t do it’s job.

“This is widely misunderstood,” says Jensen, explaining that many people still equate the pill with an abortion drug. “Even when I talk to medical students about this, I see a misunderstanding of action." The morning-after pill, he notes, has absolutely nothing to do with abortion, since it stops a pregnancy from taking place in the first place.

While hormonal birth control is still your best bet for preventing unintended pregnancy in the long-term, emergency contraception is a safe option if taken within the right time-frame. And as more research is conducted on dosage, it will likely only become more effective with time.

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Jen Gerson Uffalussy is a regular contributor to Fusion. She also writes about reproductive and sexual health/policy for Glamour, and television for The Guardian. She lives in Atlanta.