A potential biological mechanism that could explain that could explain why oral contraceptives seem to fail in preventing pregnancy among obese women has been uncovered by a research team including an Indian-origin scientist.
Although oral contraceptives appear to reach the required blood level concentrations to prevent pregnancy in such women, they may take twice as long to do so, leaving a "window of opportunity" for pregnancy every month.
"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.
Cherala said that the underlying problem is that oral contraceptives, like most drugs, are initially tested in "healthy" people, which rarely include 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. 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," Cherala said.
Cherala said that conventional oral contraceptives 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.
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 - a common measure of obesity.
The study was published in the journal Contraception.