Oral contraceptives behave differently in the bodies of obese women than in normal-weight women, new research shows, suggesting that they may not work as well in preventing pregnancy.
But more research is needed before any recommendations can be made on contraceptive use based on a woman's body mass index (BMI), a standard measure of the ratio between height and weight, Dr Alison B. Edelman of Oregon Health and Science University in Portland and her colleagues say.
There's been some evidence to suggest that the birth control pill may be less effective in obese women, but findings have not been consistent, Edelman and her team note in the journal Contraception.
Very little is known about how drug metabolism in the body is affected by obesity, they add, while obese women have been excluded from most studies of oral contraceptives.
How the study was done
To investigate whether excess weight might influence the effectiveness of the pill, the researchers assigned 20 women between the ages of 18 and 35, none of whom were using oral contraceptives, to take a birth control pill for two cycles.
Half of the women were obese, with body mass indexes above 30, while the other half were normal weight and had BMIs below 25. The women took pills containing ethinyl estradiol (synthetic oestrogen) and levonorgestrel (synthetic progesterone).
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 five days for the normal-weight women.
The longer time to reach steady-state levels of levonorgestrel may represent a "window of opportunity" for the ovary to prepare to release an egg, Edelman and her team note.
For one of the obese women, they add, it took 20 days to reach the steady state level, suggesting that ovulation may never have been adequately suppressed.
Some limitations of the study, the researchers say, are the fact that they did not include overweight women, and that they didn't directly test whether or not the women were ovulating.
"Additional studies are needed before we recommend a change in clinical practice in regard to use of oral contraception in obese women," the researchers conclude. – (Reuters Health, July 2009)
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