The research found that blood had much higher levels of hormones in women who take birth control pills compared to women who don’t.
The authors examined seven commonly prescribed birth control pills and found that four formulations more than quadruple levels of progestin, a synthetic version of the hormone progesterone.
Another formulation resulted in a 40% higher exposure to ethinyl estradiol, a synthetic version of oestrogen.
The study’s lead author, human evolutionary biologist Beverly Strassmann, stressed that birth control has greatly improved women’s lives.
But, she said, it’s also important to design birth control pills so that they don’t contribute to risk for breast cancer.
“Not enough has changed over the generations of these drugs and given how many people take hormonal birth control worldwide — millions — the pharmaceutical industry shouldn’t rest on its laurels,” she said.