"It's a 20 percent increase in risk, but the absolute risk is very low", Dr. Joann Manson said.
What those numbers mean in terms of actual women getting breast cancer who otherwise may not have is a bit less striking: there was about one extra breast cancer case diagnosed for every 7690 women who used hormonal contraception for a year.
Women who now use or recently used hormone-based contraception face a 20 percent higher risk of breast cancer, although the overall risk for most women is relatively low, a new study of 1.8 million women in Denmark has concluded.
"Progestin-only products also increased the risk of breast cancer", Morch noted. Women who used any form hormonal contraception for more than 10 years (1.38, 95% CI 1.26-1.51) had a higher risk compared to those who reported less than 1 year of use (1.09, 95% CI 0.96-1.23)(P=0.002), they wrote online in the New England Journal of Medicine.
"There were hopes that the new formulations would not increase a user's risk of breast cancer as the older formulations did", said Mia Gaudet, strategic director of breast and gynecologic cancer research at the American Cancer Society, who was not involved in the research.
Mørch explained to MedPage Today that "there was a lack of evidence on contemporary hormonal contraception and risk of breast cancer. In particular the knowledge of risk with newer progestins was sparse".
The prospective study included a cohort of 1,797,932 women from Denmark ages 15-49, participating in The Danish Sex Hormone Register Study. "Thus, it is not exclusively estrogen that increases the risk of breast cancer". Most cases of breast cancer were seen in women using oral contraceptives in their 40s.
The 20 percent increase in breast cancer risk varied by age and how long the women used hormone-based contraceptives, including pills, contraceptive patches, vaginal rings, progestin-only implants, and injections. "Depending on their reasons for using oral contraceptives, they might want to consider other options, including non-hormonal contraceptives".
Mørch told MedPage Today that she was not surprised by these findings. "Prior studies have shown similar findings".
However, hormonal birth control does lower the risk of other cancers, including ovarian cancer and endometrial cancer, and it may lower the risk of colon cancer.
In an accompanying editorial, David J. Hunter, MB, BS, ScD, of the University of Oxford in England, applauded the researchers for confirming the findings of previous literature in this area with a "much larger sample size". But, the overall risk remains small, and past research suggests oral contraceptives protect against other types of cancer.
He also added the risks associated with hormonal contraception must be weighed against the benefits. A study published Wednesday has linked newer-generation birth control pills with breast cancer; the link had already been established for older variants of hormonal contraception. The researchers tracked almost 1.8 million women starting in 1995 and compared those who purchased birth control methods with women who developed breast cancer.
The study was funded by the Novo Nordisk Foundation, a commercial foundation in Denmark that funds research to support its business interests, which include the pharmaceutical company Novo Nordisk. Mørch and Skovlund are now foundation employees.
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