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Studies of oral contraceptive (OC) failure rates in overweight and obese women compared with those in normal-weight women have yielded inconsistent results. In a pharmacokinetic study involving 15 normal-weight women (body mass index, 19.0–24.9 kg/m2) and 15 obese women (BMI, 30.0–39.9), investigators evaluated serum levels of ethinyl estradiol and levonorgestrel as well as ovarian follicular dimensions (determined by transvaginal sonography) during the third week of OC use; the OC contained 30 μg ethinyl estradiol and 150 μg levonorgestrel in a monophasic, 28-day regimen.
For ethinyl estradiol, geometric mean area under the curve (AUC) was significantly lower in obese women than in normal-weight women (P=0.04); however, trough ethinyl estradiol levels were similar in the two groups. For levonorgestrel, both AUC and trough levels were similar in both groups. Maximum ovarian follicular diameters were somewhat greater in obese women, but the difference was not significant.
Westhoff CL et al. Pharmacokinetics of a combined oral contraceptive in obese and normal-weight women. Contraception 2010 Jun; 81:474.
Comment
The key pharmacokinetic parameter that predicts contraceptive efficacy is trough progestin level. The authors' findings that trough levonorgestrel levels were similar in normal-weight and obese women, and that follicular dimensions did not differ significantly, suggest that OC failure rates in obese women are similar to those in their normal-weight counterparts. Although each group in this study had only 15 participants, that number is sufficient for drawing conclusions in pharmacokinetic studies. Clinicians should keep in mind that OC use, advancing age, and higher BMI are independent risk factors for venous thromboembolic disease. Accordingly, appropriate contraceptives for obese women who are older than 35 include progestin-only methods or intrauterine methods that do not contain estrogen or progestin (N Engl J Med 2008 358:1262).