To understand where the confusion is even coming from, we have to go all the way back to a 2011 study in the journal Contraception. Researchers found that heavier women were over three times more likely than women with normal BMIs to have failures after taking Plan B, possibly because the dose wasn't high enough to be as effective in these women.
News of the higher failure rates caused quite a freakout — and it kickstarted a chain of events that made things even more confusing. The study's findings led the makers of Norlevo, the European equivalent of Plan B, to begin investigating the effect of weight on the drug's efficacy. And in 2013, the company warned that the drug may become less effective for women over 165 lbs and completely ineffective for those over 175 lbs.
However, in 2014, the decision was reversed. After performing its own review, the European Medicines Association concluded that "these emergency contraceptives can continue to be used in women of all weights as the benefits are considered to outweigh the risks." (Translation: There's a chance it may be less effective, but it can't hurt to try.) For its part, Norlevo currently states that weight may have some effect, but the data aren't conclusive.
Birth control dosing is standardized — meaning the amount of the drug is the same no matter the weight of the patient — which is different from some other areas of medicine, where doses may be more customized. And it's interesting that as the number of women considered overweight or obese has risen (we're now at about two-thirds of women in those categories), birth control doses have stayed the same. But the data we do have suggests that weight is likely not a huge concern: A Cochrane review published in August went through the results of 12 previous studies, and the analysis suggested that weight didn't have a significant effect on the most commonly used birth control methods (including the pill, IUD, implant, and ring).
So what does all this have to do with emergency contraception? Basically, the morning-after pill is a massive dose of birth control, and it acts on the same hormones as the daily pill does. Plan B specifically, which is the most widely used oral emergency contraception option out there, provides a large dose of levonorgestrel. That's a manufactured hormone that suppresses the surge of luteinizing hormone (LH) which, in a normal menstrual cycle, signals the ovary to release an egg. "You get this hormone that starts rising really rapidly," Dr. Edelman says, "and with levonorgestrel, you have to be taking it before [the hormone] reaches the peak for it to work." Whenever you take Plan B, no matter your weight, you want to take it as soon after you have sex as possible. That's partly because if you take it after that LH peak, it won't be able to stop ovulation and it'll fail.
The thing is, though, that the study didn't really prove why or how much of an effect weight has on Plan B's efficacy. To start answering those questions, Dr. Edelman and her team recently gave 10 women levonorgestrel and measured the amount of the drug that actually made it into their systems. Their results, published in Contraception this July, show that women with BMIs considered "obese" only get about half as much of the drug in their blood compared to women with normal BMIs. Then, during their next menstrual cycle, the obese group got a double dose of levonorgestrel. That bumped their numbers up to match participants with normal BMIs.
This was just a small pilot study and didn't measure the drug's effects on ovulation, which is what really matters in terms of preventing pregnancy, says Dr. Edelman. But it definitely suggests that, for women with higher BMIs, one dose of Plan B may not be enough to actually affect ovulation.
Still, that doesn't mean that heavier women should be taking double doses of Plan B. For one thing, it's expensive. A single dose costs $50 and isn't always covered by insurance. Generic versions of Plan B do exist and are more likely to be covered by insurance, but are also somewhat expensive (around $40). But perhaps more importantly, there's even less research out there about the efficacy of double doses than single doses. In other words, we just don't know what happens if you take twice as much Plan B.
Dr. Edelman says that the very best emergency contraception option for women of all weights is actually having a copper IUD placed shortly after unprotected sex. This option is even more effective than Plan B, but going through the hoops of getting a same-day appointment and then, you know, having an IUD placed (which for some women can cause cramping), is a lot to ask of women who are already stressed about a potential unintended pregnancy.
In an ideal world, accidents wouldn't happen, and everyone would have no-fail birth control, but we know that's not always possible. In the meantime, we have to accept Plan B as the imperfect option it is. And we need to do more research to figure out the best back-up options for everyone.