Background: Tobacco smoke contains carcinogens, which may increase the risk of breast cancer (BC). Conversely, cigarette smoking also has antiestrogenic effects, which may reduce the risk of BC. The association between smoking and BC remains controversial.
Methods: Prospective cohort study of 111 140 participants of the Nurses' Health Study from 1976 to 2006 for active smoking and 36 017 women from 1982 to 2006 for passive smoking.
Results: During 3 005 863 person-years of follow-up, 8772 incident cases of invasive BC were reported. 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. Breast cancer incidence was associated with a higher quantity of current (P for trend = .02) and past (P for trend = .003) smoking, younger age at smoking initiation (P for trend = .01), longer duration of smoking (P for trend = .01), and more pack-years of smoking (P for trend = .005). Premenopausal smoking was associated with a slightly higher incidence of BC (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). Conversely, the direction of the association between postmenopausal smoking and BC was inverse (0.93; 0.85-1.02 for every increase of 20 pack-years). Passive smoking in childhood or adulthood was not associated with BC risk.
Conclusion: Active smoking, especially smoking before the first birth, may be associated with a modest increase in the risk of BC.