Study explains potential failure of oral contraceptives with obese women

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.

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 percent of all adults in the U.S. 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, an assistant professor in the College of Pharmacy at Oregon State University. "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 study was just published in the journal Contraception, by scientists from OSU, Oregon Health and Science University, University of Colorado at Denver, Oregon National Primate Research Center, and the University of Southern California. The research was supported by the National Institutes of Health.

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 percent 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."

Conventional oral contraceptives, Cherala said, are thought to be relatively "lipophilic," or tend to concentrate in fat tissue. However, 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 five 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.