A study published in the January 24, 2011 Archives of Internal Medicine [Abstract] found that cigarette smoking increased the incidence of breast cancer in premenopausal women "(HR, 1.11; 95% CI, 1.07-1.15 for every increase of 20 pack-years), especially smoking before first birth (1.18; 1.10-1.27 for every increase of 20 pack-years)."
The data are taken from the long-running Nurses' Health Study, an accrual of 30 years of biennially updated self-reports. Among the nurses in the group were 111,140 active and 36,017 passive smokers.
Among these active and passive smokers there were 8772 incidents of breast cancer during 3,005,863 person-years of follow-up, compared with 2890 during 876,996 person-years for nonsmokers.
The study was conducted by researchers from Brigham and Women's Hospital and the Harvard Medical School in Boston. According to the authors, "after adjustment for potential confounders, the hazard ratio (HR) of BC was 1.06% (95% confidence interval [CI], 1.01%-1.10%) for ever smokers relative to never smokers."
The researchers noted that the study adjusted for various factors which other studies have until now failed to do, including the levels of smoking habits, and wrote that "current and past quantity, age at which one started smoking, duration, years since quitting smoking, and pack-years of smoking after menarche were associated with a marginally higher incidence of BC."
The link between smoking and breast cancer has always been controversial, and cigarette smoking has antiestrogenic effects, which can reduce the incidence of breast cancer. This has led to various studies which claim no direct link. This study represents, to the best of the authors knowledge, the largest most comprehensive study of its kind.
For more information check out the abstract here: Arch Intern Med. 2011;171(2):125-133. doi:10.1001/archinternmed.2010.503
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• New study: Smoking presents greater hazard to women
• Stopping Smoking May Increase Breast Cancer Survival