Active and passive smoking and risk of breast cancer: a meta-analysis.

Studies on active and passive tobacco smoking and breast cancer have found inconsistent results. A meta-analysis of observational studies on tobacco smoking and breast cancer occurrence was conducted based on systematic searches for studies with retrospective (case-control) and prospective (cohort) designs. Eligible studies were identified, and relative risk measurements were extracted for active and passive tobacco exposures. Random-effects meta-analyses were used to compute summary relative risks (SRR). Heterogeneity of results between studies was evaluated using the (I (2)) statistics. For ever active smoking, in 27 prospective studies, the SRR for breast cancer was 1.10 (95 % CI [1.09-1.12]) with no heterogeneity (I (2) = 0 %). In 44 retrospective studies, the SRR was 1.08 (95 % CI [1.02-1.14]) with high heterogeneity (I (2) = 59 %). SRRs for current active smoking were 1.13 (95 % CI [1.09-1.17]) in 27 prospective studies and 1.08 (95 % CI [0.97-1.20]) in 22 retrospective studies. The results were stable across different subgroup analyses, notably pre/post-menopause, alcohol consumption adjustments, including/excluding passive smokers from the referent group. For ever passive smoking, in 11 prospective studies, the SRR for breast cancer was 1.07 (95 % CI [1.02-1.13]) with no heterogeneity (I (2) = 1 %). In 20 retrospective studies, the SRR was 1.30 (95 % CI [1.10-1.54]) with high heterogeneity (I (2) = 74 %). Too few prospective studies were available for meaningful subgroup analyses. There is consistent evidence for a moderate increase in the risk of breast cancer in women who smoke tobacco. The evidence for a moderate increase in risk with passive smoking is more substantial than a few years ago.