title: Breast cancer risk among women who start smoking as teenagers.
abstract: OBJECTIVE
  To examine the effect of smoking on breast cancer risk in a large population-based cohort of women, many of whom started smoking as teenagers.
  
  
  METHODS
  We followed 102,098 women, ages 30 to 50 years, completing a mailed questionnaire at recruitment to the Norwegian-Swedish Cohort Study in 1991/1992, through December 2000. We used Cox proportional hazard regression models to estimate relative risk (RR) of breast cancer associated with different measures of smoking initiation, duration, and intensity adjusting for confounding variables. We conducted analyses on the entire study population, among women who had smoked for at least 20 years, among nondrinkers, and separately for each country.
  
  
  RESULTS
  Altogether, 1,240 women were diagnosed with incident, invasive breast cancer. Compared with never smokers, women who smoked for at least 20 years and who smoked 10 cigarettes or more daily had a RR of 1.34 (95% CI, 1.06-1.70). Likewise, those who initiated smoking prior to their first birth (1.27, 1.00-1.62), before menarche (1.39, 1.03-1.87), or before age 15 (1.48, 1.03-2.13) had an increased risk. In contrast, women who had smoked for at least 20 years, but started after their first birth, did not experience an increased breast cancer risk. The increased RR associated with smoking was observed among nondrinkers of alcohol, women with and without a family history of breast cancer, premenopausal and postmenopausal women, and in both countries.
  
  
  CONCLUSIONS
  Our results support the notion that women who start smoking as teenagers and continue to smoke for at least 20 years may increase their breast cancer risk.
authors.affiliation: Inger T. Gram, Institute of Community Medicine, University of Tromsö, N-9037 Tromsö, Norway. inger.gram@ism.uit.no
authors.forename: Inger T
authors.initials: IT
authors.lastname: Gram
authors.forename: Tonje
authors.initials: T
authors.lastname: Braaten
authors.forename: Paul D
authors.initials: PD
authors.lastname: Terry
authors.forename: Annie J
authors.initials: AJ
authors.lastname: Sasco
authors.forename: Hans-Olov
authors.initials: HO
authors.lastname: Adami
authors.forename: Eiliv
authors.initials: E
authors.lastname: Lund
authors.forename: Elisabete
authors.initials: E
authors.lastname: Weiderpass
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D001943:Breast Neoplasms / Q000453:epidemiology*; D015331:Cohort Studies; D015986:Confounding Factors, Epidemiologic; D005260:Female; D006801:Humans; D015994:Incidence; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D016016:Proportional Hazards Models; D012306:Risk; D012907:Smoking / Q000009:adverse effects*; D011795:Surveys and Questionnaires; D013548:Sweden / Q000453:epidemiology; D013997:Time Factors