A new statistical analysis finds that women under age 50 who were diagnosed with breast cancer were also more likely to have recently been on some versions of the Pill.
The increased cancer risk still translates to less than a one per cent chance of developing breast cancer for most younger women, researchers emphasise, 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 tumours 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: 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 analysed 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 tumours, 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 per cent of the comparison group had filled prescriptions for oral contraceptives, compared to 13 per cent of women diagnosed with breast cancer, according to pharmaceutical records.
The authors calculate that taking oral contraceptives is therefore associated with a 50 per cent increase in risk of breast cancer for women like those in the study, who were between ages 20 and 49.
Less than one per cent of women will get breast cancer before age 40, according to the American Cancer Society, and even with a 50 per cent relative increase that number would still be under one per cent.
For this type of study, a 50 per cent increase in risk is actually quite low and can generally be dismissed as bias or random chance, Dr. David A. Grimes told Reuters Health by phone.
Grimes is a clinical professor in the department of Obstetrics and Gynecology at UNC School of Medicine in Wilmington, North Carolina.
"Weak associations, consistent with noise and not signal, were the overall finding," he said.
The new results are not important for women or doctors, Grimes said. Other, better studies have found no increase in breast cancer risk with birth control, which is essential for women's health, he said.
The new study did not include interviews with women nor did it assess whether they actually took the pills, only if they had a prescription on file at the time, he pointed out.
The bottom line for women is that birth control pills are very safe, he said.
Beaber and her co-authors acknowledge that their results "should be interpreted cautiously" and further studies with a larger group of women are needed to confirm the findings.
"It is important to remember that breast cancer is rare among young women and that there are other established health benefits associated with oral contraceptive use," Beaber said.
Oral contraception helps protects against pelvic inflammatory disease, lowers the risk of ovarian and endometrial cancers and colorectal cancers and helps maintain bone density.