FYI: Why Doesn't Plan B Work For Heavier Women?

Even a double dose of the morning-after contraception isn't likely to work.

Pillbox, U.S. National Library of Medicine
Front and Back of a Tablet of Plan B One-Step, an Emergency ContraceptivePillbox, U.S. National Library of Medicine

If you're a lady—or even if you're not—you might have heard already. Mother Jones reported on Monday that a European manufacturer of a pill identical to Plan B One-Step, the U.S.' best-known "morning-after pill," is warning patients that its product doesn't work well for women who weigh more than 165 pounds. And it doesn't work at all for women over 176 pounds. The average American woman weighs 166 pounds. (Mother Jones also published a story today clarifying that the strongest evidence ties emergency contraceptive's effectiveness directly to weight, not to body mass index or BMI, which is a measure of weight in proportion to height. Here, we'll consider weight and BMI interchangeable, although they aren't exactly so.)

Interestingly, researchers suspect it's not just a matter of getting heavier women to take more of the Plan B chemical, called levonorgestrel. There have been no studies to check whether more levonorgestrel would work for heavier women, but, in general, the contraceptive scientists Popular Science spoke with thought that that is unlikely to solve the problem. (If you'd like to try anyway, levonorgestrel is safe to take in a double dose.) With levonorgestrel, what you put in isn't directly related to what you get out. In other words, when you take a double dose of levonorgestrel, the amount of the chemical that shows up in your bloodstream doesn't double. That's true no matter what you weigh.

This phenomenon is called non-linear pharmacokinetics, and it's pretty common in drugs. "It is a nightmare for pharma companies as such phenomenon will make their drugs less appealing to clinical practice," Ganesh Cherala, a pharmaceutical scientist at the Oregon Health & Science University, wrote in an email. "They usually weed out drugs with high potential for non-linear pharmacokinetics." Levonorgestrel-based emergency contraceptives made their way to doctor's offices anyway because the original studies on contraceptives of all kinds were usually done in normal BMI women.

Cherala works with obstetrician-gynecologist Alison Edelman, who has published numerous studies on the effectiveness of contraceptives in women of different weights. In addition to levonorgestrel's non-linear pharmokinetics, Edelman and her collaborators are trying to figure out whether heavier women's bodies treat levonorgestrel differently. So far, they've determined that in obese women, levonorgestrel leaves the bloodstream at a different rate than in normal-weight women.

Levonorgestrel may also enter the bloodstream at a different rate in obese women. Patients take the morning-after pill by swallowing it, which means it passes through the liver before it reaches the bloodstream. This happens to everybody, no matter her weight, but overweight and obese women's livers also happen to create more drug-metabolizing enzymes than normal-weight women's livers, Cherala says. That means more levonorgestrel may be chewed up in an obese women's liver before it ever makes it to her bloodstream, which would carry it to her reproductive organs, where it does its work.

So heavier women have a lot of things working against them when they try to use levonorgestrel as an emergency contraceptive. That doesn't mean they're totally out of luck. They may get copper intrauterine devices fitted as an emergency contraceptive. IUDs aren't affected by weight, Edelman says. They may also use an emergency contraceptive called ulipristal acetate, brand name Ella. The same study that found levonorgestrel doesn't work well for women over 165 pounds also found that ulipristal is not as affected by weight, although it, too, seems to lose its efficacy in women over 194 pounds (88 kilograms) or with BMIs over 35. Bonus: Copper IUDs and ulipristal are actually slightly more effective as emergency contraceptives than levonorgestrel at any given weight.

Nothing is quite as convenient in the U.S. as levonorgestrel, however. It's the only emergency contraceptive available over-the-counter to anybody of any age. IUDs must be professionally fitted, and ulipristal requires a prescription for patients of all ages.