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| |  Study Explains Potential Failure of Oral Contraceptives With Obese Women PORTLAND, Ore -- July 15, 2009 -- Researchers have identified a potential biological mechanism that could explain why oral contraceptives may be less effective at preventing pregnancy in obese women, as some epidemiological studies have indicated. The study is published in the journal Contraception. Although conventional oral contraceptives appear to eventually reach the effective blood concentrations needed in the body to prevent conception in obese women, it appears to take twice as long, leaving a window of opportunity every month where the contraceptive may not be at a high enough level to prevent a pregnancy. The findings are of particular importance, researchers noted in their study, because about 30% of all adults in the United States are obese and the birth control pill is one of the most popular forms of contraception in the nation. "We don't have enough data yet to recommend that physicians change their clinical practice for use of oral contraceptives with patients who are very overweight," said Ganesh Cherala, College of Pharmacy, Oregon State University, Portland, Oregon. "However, until more studies are done, women may wish to consult with their physicians about this issue and consider a backup method of contraception at some times of the month." The underlying problem, Cherala said, is that oral contraceptives, like most drugs, are initially tested in healthy people, which rarely includes people who are more than 130% of their ideal body weight. "When we first test drugs for safety and efficacy, we generally do not include people with a high body mass index," Cherala said. "But body weight and amounts of fat can seriously change the pharmacokinetics, or way drugs act and are processed in the body. There's a growing awareness that we need to more carefully consider obesity and other factors that affect drug absorption, distribution, metabolism and other factors." The researchers in this study said they were somewhat surprised to find that the affinity of these drugs for fat tissue was not significantly different between obese and normal body weight subjects. Rather, the researchers found that contraceptive drug levels in both obese women and those of normal weight eventually were about the same, but it took longer to achieve that level in very overweight women. The study showed it took an average of about 5 days for the drugs to achieve their maximum concentration in women of normal weight, an average of 10 days for obese women, and even longer than that for some individuals. One woman in the study took more than 20 days to reach a steady state drug concentration. Women of normal weight who follow their oral contraceptive directions should have appropriate protection against pregnancy. But the delay in reaching a steady state drug concentration raises questions about how well oral contraceptives may work for obese women. Increasing the drug dosage might help address this issue, Cherala said, but also adds other health concerns. In fact, the researchers noted in their report that many clinicians actually prescribe lower-dose oral contraceptives to obese patients in an effort to decrease their risk of venous thrombosis. The study was done with 20 women of ages 18 to 35, all of them healthy and seeking contraception, 10 of whom were of normal weight and 10 with a body mass index of more than 30. SOURCE: Oregon State University
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