The “Plan B” contraceptive pill is the most affordable and convenient form of emergency contraception for women in the U.S., yet it has a remarkable failure rate in larger women.

Physicians may want to think twice before suggesting the “Plan B” pill to patients in need of emergency contraception (EC).

The results of a large European study show that the popular form of EC, levonorgestrel 1.5 mg, (Plan B pill) loses its potency in women weighing about 165 pounds and does not work at all in women weighing 175 pounds or more. Since the average weight of a woman in the U.S. is 166.2 pounds according to the U.S. Centers for Disease Control and Prevention (CDC), women have few options but to take a gamble on the only approved form of over-the-counter emergency contraception.

The study originally set out to compare the effectiveness of different types of EC. It studied the outcomes of the levonorgestrel “morning after” pill and the ulipristal acetate pill known as “Ella” in about 2,000 women. The lead author, Anna Glasier, MD, honorary professor of Obstetrics and Gynecology at the University of Edinburgh, in Scotland, and a world-renowned expert on EC, decided the data would be more practical if it could be used to identify at-risk patients.

“Somewhere between 90% to 95% of women who take emergency contraception do not appear to be at great risk of pregnancy, so if you could work out who was genuinely at risk, it might help you as a clinician,” she explains.

She and her team conducted a meta-analysis and found a worrisome pattern: Levonorgestrel consistently failed in heavier women. Generally, levonorgestrel prevents about 50% of pregnancies that would have occurred without intervention, but when Glasier factored in weight, the effectiveness changed drastically. Women with a body mass index (BMI) of 25 or higher experienced decreased efficacy, and the pill stopped working entirely in obese women with a BMI of 30 or higher. In fact, obese women that took levonorgestrel were slightly more likely to become pregnant, 5.8%, than women who did not take EC at all, 5.6%. The number of obese women included in the study was small, but still statistically significant.

These findings elicited a change in the patient information packets of the European levonorgestrel EC that reflects the drug’s impotence in overweight and obese women. The FDA is currently reviewing the evidence and considering a change to U.S. labels, but most women are still unaware of the drug’s limited efficacy. Reports by National Public Radio state that physicians treating overweight and obese patients regularly see women who became pregnant after taking the Plan B pill.

This glaring issue was overlooked because the original research used to bring the Plan B pill to market did not focus on a representative sample of women in the U.S. and Europe. It obtained approval from the FDA based on data from the World Health Organization in which the average BMI of participants was too small to see the effects of weight. “If you look at the first study, they reported the BMI and the mean was 22 with a standard deviation of plus or minus three. So these were not, by and large, very heavy women,” Glasier explains.

More Weight, Less Efficacy

Several theories exist as to why the levonorgestrel pill fails in women over a certain weight: the dilution of the steroids in a larger blood volume; hormones becoming sequestered in fat cells; or the drug might be metabolized differently in a larger person. Yet nothing is definitive. “There is a lot of evidence that certain contraceptives are less effective in heavier women, although the studies haven’t really been of really good quality. None, including ours, have been designed to look specifically at the relationship between effectiveness and weight,” Glasier says.

Quite possibly, a larger patient simply needs a larger dose of the drug. “It is not really surprising because if you do studies on animals you dose them on a weight basis; however many milligrams per kilogram. The only reason that we don’t do that with human medication is because it would just be so horribly complicated,” Glasier continues.

The only consolation to the dismal success rate of levonorgestrel is that resulting infants are not adversely affected if a patient still becomes pregnant. Glasier insists there is no evidence for concern over a baby’s future health due to a mother ingesting Plan B or the Ella pill. That said, women should definitely be advised to consider other options.

Ella proved to be more effective than levonorgestrel regardless of weight and definitely performed better on overweight and obese women. Ella bested Plan B by half, with about 50% fewer pregnancies than those taking levonorgestrel. For both drugs, pregnancy risk is elevated by additional factors unrelated to a patient’s weight. Women who had intercourse around the time of ovulation had a fourfold increase in the likelihood of pregnancy compared to women having sex outside the window of fertility. Those who had unprotected sex after using either pill type were also more likely to get pregnant.

The IUD Option

Due to these mitigating factors, Glasier recommends the copper intrauterine device (IUD) as a first line of defense. It offers a 95% success rate as emergency contraception, making it the most effective option by far. “The big advantage, of course, is that once it’s in place you can keep it in place and it is a highly effective ongoing contraceptive until you want to get pregnant and fertility comes back to normal within a week.”

Ideally, of course, women should avoid the need for EC entirely by getting a copper or Mirena IUD or a hormonal implant before an emergency arises. Glasier describes these options as “independent of compliance,” meaning that patients do not need to remember to take a daily pill or follow other such instructions. But when it is too late for preventative steps, the copper IUD is the way to go.

Unfortunately, an IUD is not available from pharmacies and a lot of women do not like the idea of having a foreign object inserted into their cervix. In the U.S., it is much more expensive than taking an emergency contraception tablet. A lack of health insurance coverage could make the costs prohibitive, and the extra steps involved may deter some patients from taking action.

The Ella pill also requires a visit to a doctor and a prescription, which prohibits some women from access. Glasier still recommends this option over Plan B if a woman is not willing or able to obtain a copper IUD.

For now the most affordable and convenient form of emergency contraception for women in the U.S. is overthe-counter levonorgestrel, and it is for sale without any warning of its ineffectuality in larger women. Those who still become pregnant will have to think of a “Plan C.”

— Mapes is a Washington D.C.–based freelance writer.

She wrote about diabetes social networks in the December issue.