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.'

The study was published recently in the journal Contraception.