Obese women starting to take the birth control pill should also use an alternative method of contraception for at least 10 days, and perhaps for as long as 20 days because it takes that long for effective levels of the drug to build up in the blood, Oregon researchers reported today. In women of normal weight, such a buildup only requires about five days, they found.
Epidemiological research has suggested that the pill does not work as well in obese women -- those with a body mass index (BMI) higher than 30. For a woman 5 foot, 6 inches tall, that is a weight greater than 190 pounds. Some experts had thought the problem was that the hormones in the pill were selectively deposited in fat cells and didn't get into the blood stream, but pharmacologist Ganesh Cherala of Oregon State University and his colleagues reported in the journal Contraception that that is not the case. Instead, it simply takes longer for the drug to build up in the blood stream.
The researchers studied 20 women, ages 18 to 35, all healthy and seeking contraception. Ten were of normal weight and 10 had a BMI above 30. The women stayed in the clinic for 48 hours at the beginning and end of the birth control cycle and blood levels of reproductive hormones were measured during those periods and twice per week on an outpatient basis.
They found that the drugs took an average of about five days to achieve their maximum levels in the women of normal weight, but an average of 10 days in the heavier women. Some took even longer -- one woman took more than 20 days. The situation can be further complicated, the team noted, because many physicians prescribe lower-than-normal doses of the pill to obese women in an effort to reduce their risk of developing venous thrombosis -- blood clots in their legs and elsewhere that can increase the risk of heart attack and stroke.
Dr. Alison Edelman of the Oregon Health and Science University, senior author of the study, cautioned that the team did not study enough women to make definitive clinical recommendations, "other than choosing a contraceptive option that works better for both normal weight women and obese women, like an intrauterine device."
-- Thomas H. Maugh II