Cigarette smoking and the incidence of breast cancer.
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.


CONCLUSIONS
Active smoking, especially smoking before the first birth, may be associated with a modest increase in the risk of BC.
Obstetrics and Gynecology Epidemiology Center, Department of Obstetrics and Gynecology, Brigham and Women's Hospital, Harvard Medical School, Boston, MA 02115, USA. n2fei@channing.harvard.edu
Fei
F
Xue
Walter C
WC
Willett
Bernard A
BA
Rosner
Susan E
SE
Hankinson
Karin B
KB
Michels
D000428:Alcohol Drinking / Q000453:epidemiology; D015992:Body Mass Index; D001941:Breast Diseases / Q000453:epidemiology; D001943:Breast Neoplasms / Q000453:epidemiology* / Q000209:etiology; D015331:Cohort Studies; D003276:Contraceptives, Oral / Q000008:administration & dosage; D005260:Female; D006801:Humans; D015994:Incidence; D008875:Middle Aged; D017698:Postmenopause; D011446:Prospective Studies; D012307:Risk Factors; D012907:Smoking / Q000009:adverse effects / Q000453:epidemiology*; D014028:Tobacco Smoke Pollution / Q000009:adverse effects
D003276:Contraceptives, Oral; D014028:Tobacco Smoke Pollution
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