Researchers from the University of Copenhagen followed 1.8 million Danish women between the ages of 15 and 50 for over a decade.
They found that for every 100,000 women using the combined pill, progesterone-only pill, vaginal ring or IUD, an additional 13 cases of the disease occurred each year. This means that out of 100,000 women on birth control, there will be 66 diagnoses annually, compared with 55 among non-users.
The level of risk was also shown to be dependant on the length of time each woman had taken the contraception – with a 9 per cent increase in those who had been using it for less than 12 months, and a 38 per cent increase if it had been longer than 10 years. Interestingly, after the women switched to non-hormonal methods, this gradually declined with time and the risk completely disappeared five to ten years later.
“This is an important study because we had no idea how the modern-day pills compared to the old-fashioned pills in terms of breast cancer risk, and we didn’t know anything about IUDs,” Dr Marisa Weiss, an oncologist who founded breastcancer.org, told the New York Times.
“Gynecologists just assumed that a lower dose of hormone meant a lower risk of cancer. But the same elevated risk is there.”
“It’s small but it’s measurable,” she continued. “if you add up all of the millions of women taking the pill it is a significant public health concern.”
However, speaking with CNN, David Hunter - a professor epidemiology and medicine at the Nuffield Department of Population Health – explained that it is important to take these findings into perspective.
He pointed out that many hormonal contraceptives, in particular, oral contraceptives are also linked to a lower risk of ovarian, endometrial and colorectal cancers later in life.
“The benefits (against these other cancers) persists for one or two decades,” he told the publication. “[So] overall, it may be more beneficial.”