Oral contraceptives behave differently in the bodies of obese women, suggesting that they may not work as well in preventing pregnancy.
Very little is known about how drug metabolism in the body is affected by obesity, and obese women have been excluded from most studies of oral contraceptives. To investigate whether excess weight might influence the effectiveness of the pill, researchers assigned 20 women to take a birth control pill for two cycles. Half of the women were obese.
For the obese women, it took about 10 days of taking the pill for their blood levels of levonorgestrel to reach the optimum steady-state concentration for suppressing ovulation, compared to about 5 days for the normal-weight women.