The increased cancer risk still translates to less than
a one percent chance of developing breast cancer for
most younger women, researchers emphasize, so the
results should not outweigh the many benefits of taking
oral contraceptives.These results are not enough to change clinical practice or to discourage any women from taking birth control pills, said lead study author Elizabeth F. Beaber, of the Fred Hutchinson Cancer Research Center in Seattle, Washington.
Some past research suggests that the hormones in birth control pills could “feed” hormone-sensitive tumors and thereby raise younger women’s risk of a breast cancer diagnosis, or of developing more aggressive cancers (see Reuters Health article of February 26, 2013 here: http://reut.rs/1s8tM44).
But birth control pills have evolved over the decades since their introduction and the hormone doses they contain have dropped steadily, so many studies are based on data for formulations that are no longer used, Beaber and her colleagues point out in the journal Cancer Research.
To examine the risk in a group of women more recently on birth control pills, Beaber’s team analyzed data from a large healthcare delivery system, tracking birth control pill prescriptions and breast cancer diagnoses.
The researchers compared 1,102 women diagnosed with invasive breast cancer between 1990 and 2009 with 21,952 women without cancer who were of similar age.
Women who had taken oral contraception during the past year, according to pharmacy records, were more likely to be in the cancer group than those who had never taken birth control pills or who had taken them more than a year prior.
Contraceptives with higher doses of estrogen or progestin were more strongly associated with increased cancer risk.
"Use of formulations with high dose estrogen, ethynodiol diacetate (synthetic progestin), and specific triphasic oral contraceptives in the past year was associated with an increased breast cancer risk in our study, while other formulations, including low dose estrogen oral contraceptives, did not appear to be associated with an elevated risk," Beaber told Reuters Health.
Women who had used high-estrogen pills or those with high synthetic progestin in the past year were about 2.6 times as likely as women who had not used oral contraceptives to be diagnosed with breast cancer. Women who used norethindrone (Aygestin), a progestin-only pill, in triphasic dosing were 3.1 times more likely to have a breast cancer diagnosis.
Overall, the risk was slightly higher for hormone-sensitive cancers than for other types of tumors, but that result was not statistically significant, meaning it could have been due to chance.
Women in the cancer group had more family history of cancer than the comparison group, and may have differed in other ways that the authors could not account for.
More research is required to determine why and how different birth control options might affect breast cancer risk, Beaber said.
Nine percent of the comparison group had filled prescriptions for oral contraceptives, compared to 13 percent of women diagnosed with breast cancer, according to pharmaceutical records.
