Today’s myth: that the pill (because there is no other “pill,” right?) is less effective for overweight and obese women compared with women who are normal weight.
Knowing if there is an association between pill failure and obesity is important given that 60% of reproductive aged women are over weight or obese.
You may not know that medicine is a cesspool of urban myth. It’s an end product of people not quite remembering what they read, making assumptions, and bad studies. Or sometimes someone wrote some crap in a textbook back in 1960 and that same error is perpetuated with each edition because, I don’t know, it sounded good then and no one bothered to check it out since.
But that’s why evidenced-based medicine is important. Because patients deserve facts, not myths. It’s medicine folks, not witchcraft.
I suspect the “pill is less effective for obese women” myth is born from a few sources. First of all, some medications do need to be dosed based on body weight. Secondly, clinical trails for the birth control patch indicated an increased pregnancy rate for women weighing over 90 kg. And then thirdly, once the Ortho Evra/90 kg association was made people started looking into the obesity/birth control pill connection. And the original retrospective studies did show an association between body weight and contraceptive failure.(1)
Fortunately, there are two prospective clinical trials (better than retrospective studies…if you want those references they are Dinger et al and Westhoff et al) that tell us there is no association between body weight and pill failure.
An even more recent prospective study confirms that even obese women taking the lowest dose of birth control pill (20 mcg of ethinyl estradiol) get the same degree of ovarian suppression as women who are of normal weight: in both groups 97% do not ovulate. (2) (As an aside, understanding that 2.7% of women ovulate on the pill helps you to see the 2% failure rate).
But this study did show one important difference between obese and normal weight women, obese women were more likely to be inconsistent pill takers: 90% of normal weight women took the pill every day as directed as compared with 72% of obese women. Obviously, taking the pill every day is essential for it to work correctly. When women didn’t take the pill every day as directed, almost 40% ovulated. The study couldn’t answer why obese women were not as good at taking their pills every day.
There are many factors that affect pill compliance: age, education, and motivation to not get pregnant are all up there, and so now it seems that obesity might also be a factor. I say might because this is one prospective study. While it is a high quality study, a randomized clinical trial from a well-respected researcher, it is always nice to have a confirmatory study from another part of the country.
While the birth control pill is equally effective for women of all sizes, spending an extra minute discussing the importance of taking the pill every day is important and offering the full range of contraceptive methods as alternatives is just good practice. For every patient.
1. Holt VL, Cushing-Haugen KL, Daling JR. Body weight and risk of oral contraceptive failure. Obstet Gynecol 2002.
2. Westhoff et al. Ovarian suppression in normal-weight and obese women during oral contraceptive use. Obstet Gynecol 2010