Failure of the Pill in Obese Women

Oral contraceptives are less effective at preventing pregnancy in obese women. Researchers from Oregon State University say they’ve identified a biological mechanism that could explain the failure.

The study, published in the journal Contraception, found that oral contraceptives eventually reach the effective blood concentrations needed in the body to prevent pregnancy in obese women, but it may take twice as long. That would allow for a “window of opportunity” each month. During that time, the contraceptive may not be at a high enough level to prevent a pregnancy.

One problem is that drugs aren’t initially tested for effectiveness on people with a high body mass index. Researchers and doctors are becoming more aware that obesity may affect drug absorption and other factors. Body weight may also complicate matters in other ways. The researchers noted that many obese women are prescribed lower-dose oral contraceptives in an effort to decrease their risk of venous thrombosis, blood clots that may increase the risk of stroke and heart attacks.

“Although our research has found this interaction between obese women and oral contraceptives, we don’t have enough information yet to recommend changes in clinical practice, other than choosing a contraceptive option that works better for both normal weight and obese women, like an intrauterine device,” said Dr. Alison Edelman, lead author of the study.

Researchers suggest obese that women may wish to consult with their doctors about this issue and perhaps consider a backup method of contraception at some times of the month.

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