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Women taking certain formulations of birth control pills could face a 50 percent or higher increased risk of breast cancer than those not using oral contraceptives, according to a study by Fred Hutchinson Cancer Research Center scientists.
Some versions of the drugs used to prevent pregnancy – particularly those with high doses of estrogen, some types of progestin and certain dosing schedules – appeared to boost the risk, according to a review of 20 years of health system electronic pharmacy records.
But the alarming-sounding results don’t mean that anyone should stop taking birth control pills or change their brands – at least not without first talking a health care provider, said Dr. Owen Montgomery, a spokesman for the American College of Obstetricians and Gynecologists.
Most women don’t take the high-risk pills, he noted, and low-dose estrogen posed no increased risk, according to the study published Friday in the journal Cancer Research.
“I can tell you, I haven’t seen a woman in 20 years on a high-dose pill,” said Montgomery, who also heads the Department of Obstetrics and Gynecology at Drexel University College of Medicine.
Women should still carefully weigh the risks of using oral contraceptives against the many benefits they provide, said Dr. Elisabeth F. Beaber, the Fred Hutch staff scientist who led the study.
“Our study results need to be interpreted cautiously,” she said. “This is an important contribution, but it is not yet at the scale where it is … changing any clinical recommendations.”
About 10.5 million women in the U.S. use birth control pills to prevent pregnancy, according to 2010 figures from the Guttmacher Institute, which tracks reproductive trends.
Previous studies have shown oral contraceptives were likely to contribute to a slightly higher risk of breast cancer, but those reviews relied on women’s own reports of the types of birth control pills they were taking and did not examine newest generation of formulations, the researchers said.
Beaber and a team from the University of Washington and the Group Health Research Institute analyzed birth control prescription records from a large database of women enrolled in Group Health Cooperative, a Seattle-area health care system. They looked at 1,102 women aged 20 to 49 who developed breast cancer from 1990 to 2009 and compared their use of oral contraceptives with 21,952 women who remained cancer-free.
Compared with the control group, the women who had used birth control pills in the previous year had a 50 percent higher risk overall than women who had either never used the drugs or had used them in the past, Beaber said.
Birth control pills containing high-dose estrogen, 50 micrograms or more, boosted breast cancer risk nearly threefold and those that contained moderate-dose estrogen, 30 to 35 micrograms, increased the chances about 1.6 times, the researchers found.
In contrast, pills that delivered low-dose estrogen, 20 micrograms, didn’t increase the risk, the study showed.
Among other culprits were pills that contained a type of progestin called ethynodiol diacetate and triphasic pills – those that deliver drugs in multiple doses or phases – that contained norethindrone.
Most of the cancer-free women – some 78 percent – who took birth control pills in the past year received prescriptions for moderate-dose estrogen compared with 24 percent who got the low-dose drugs – and only 1 percent received scripts for high-estrogen pills.
That’s probably changed even more since the study’s end date in 2009, Montgomery said. “The bulk of the medications prescribed in 2014 overwhelmingly are likely to be in the low-dose or moderate-dose category,” he added.
The researchers didn’t list brand names of the birth control pills most associated with increased risk of breast cancer, partly because there are so many formulations and they change so frequently, Beaber said.
Instead, they noted, for instance, that drugs containing ethynodiol diacetate raised the risk of breast cancer by 2.6 times. Zovia is one brand that contains that formulation, drug records show.
The drugs involved in the analysis include some of the most widely prescribed birth control pills as tracked by IMS Health, the largest vendor of U.S. prescribing data.
The top-prescribed birth control pill is Sprintec 28, with some 4.8 million scripts issued in 2013. The drug includes norgestimate and ethinyl estradiol, which appeared to raise the risk of cancer 20 percent, the study showed.
But the fifth most-prescribed pill, Loestrin 24 Fe, with more than 3 million prescriptions issued last year, contains norethindrone acetate, which increased breast cancer risk by 60 percent, the study found.
Still it’s too soon to say whether the risks would hold up in a different population or a larger group, Beaber said. The results of the study, which was funded by the National Cancer Institute, Fred Hutch and the state of Washington, would need to be replicated before they could translate to any changes in the way women use birth control.
“In addition, our study population was ages 20 to 49 and breast cancer is rare among those women,” she noted.
For such women, birth control pills offer established health benefits including reproductive planning, regulation of menstrual cycles and a lower risk of benign breast conditions.
Women worried about the link between oral contraceptives and breast cancer should call their doctors, Montgomery said.
“Talk to your provider about the type of birth control you’re using and make sure it meets your needs in light of this new information,” he said.
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JoNel Aleccia is a staff writer at Fred Hutchinson Cancer Research Center. From 2008 to 2014, she was a national health reporter for NBC News and msnbc.com and a reporter, editor and columnist for more than two decades at newspapers in the Northwest. Reach her at email@example.com.
Solid tumors, such as those of the breast, are the focus of Solid Tumor Translational Research, a network comprised of Fred Hutchinson Cancer Research Center, UW Medicine and Seattle Cancer Care Alliance. STTR is bridging laboratory sciences and patient care to provide the most precise treatment options for patients with solid tumor cancers.
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