There was optimism that newer, low-dose contraceptives would lower the breast cancer risk, but these results have dashed those hopes, said Gaudet, who wasn't involved in the research.
So the message to women taking hormonal contraceptives-no need to stop immediately.
However, the overall absolute increase for breast cancer cases was relatively small, marked by an increased of around one new breast cancer case per 7,690 current and recent users of hormonal contraception (13 per 100,000 person-years, 95% CI 10-16). After all, it means that almost a quarter of American women are doing something that might increase their risk of developing breast cancer by a third-in theory.
While contraceptive drugs that contain oestrogen have always been suspected of increasing the likelihood of breast cancer, researchers had expected smaller doses of the hormone, often combined with the drug progestin, would be safer, said Lina Morch, an epidemiologist at Copenhagen University Hospital who led a study analysing the records of 1.8 million women in Denmark. Women who had used hormonal birth control for less than a year had only a 9 percent increase in their relative risk, while women who had used birth control for more than 10 years had a 38 percent increase.
"And there is also the reassuring thought that oral contraceptive use may decrease the risk of ovarian and endometrial cancer". The link with cancer risk exists not only for older generations of hormonal contraceptives but also for the products that many women use today, according to a paper published Wednesday in the New England Journal of Medicine.
Researchers analyzed health records of 1.8 million women, ages 15 to 49, in Denmark where a national health care system allows linking up large databases of prescription histories, cancer diagnoses and other information.
"Progestin-only products also increased the risk of breast cancer", Morch noted.
Newer formulations of birth control pills appear the carry the same risk of breast cancer as older versions.
They found that women taking estrogen/progestin birth control pills have about a 20 percent increased risk of breast cancer.
One thing reiterated by every doctor Newsweek spoke to: Women who are anxious about how their contraception might increase their risk of breast cancer should speak with their health care provider.
They include smoking, obesity, starting menstruation early, having children late in life or not at all and not breastfeeding.
Two types of birth control pills are sold in the USA - one that combines synthetic versions of the hormones estrogen and progesterone, and the "minipill" that only delivers progestin, a synthetic formulation of progesterone.
"For many women, hormonal contraception-the pill, the patch, the ring, IUDs, and the implant-is among the most safe, effective and accessible options available", said the American College of Obstetricians and Gynecologists's vice president of practice, Dr. Chris Zahn.
"Estrogen has been the primary focus of breast cancer research in general, and so we know much more about it than we do progesterone", Gaudet said. For a 20-year-old woman, for example, the probability of developing breast cancer in the next 10 years is 0.06 per cent, or 1 in 1,732, according to breastcancer.org.
In Denmark, older women who have completed their families are most likely to use IUDs, including those containing hormones, and they are already more likely to develop breast cancer because of their age, Mørch said. That roughly translates to a 12 percent lifetime risk for a woman, although many factors affect breast cancer risk.
"No type of hormone contraceptive is risk-free unfortunately", said lead author Lina Morch of Copenhagen University Hospital. Condoms and diaphragms do not deliver hormones.
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