The effect of reducing the number of cigarettes smoked on risk of lung cancer, COPD, cardiovascular disease and FEV1 - A review

Searches identified 14 studies investigating effects of reducing cigarette consumption on lung cancer, CVD, COPD or FEV1 decline. Three were case-control studies, six cohort studies, and five follow-up studies of FEV1. Six studies consistently reported lower lung cancer risk in reducers. Compared to non-reducers, meta-analysis (random-effects) showed significantly lower risk (RR 0.81, 95% CI 0.74-0.88 for any reduction, and RR 0.78, 0.66-0.92 for the greatest reduction), with no between-study heterogeneity. Four cohort studies presented CVD results, the combined RR for any reduction being a non-significant 0.93 (0.84-1.03). An effect of reduction was not consistently seen for COPD or FEV1 decline. Four cohort studies presented all-cause mortality results, the combined RR of 0.92 (0.85-1.01) being non-significant. The RR of 0.95 (0.88-1.02) for total smoking-related cancer, from three studies, was also non-significant. The evidence has various weaknesses; few studies, few cases in reducers in some studies, limited dose-response data, incomplete adjustment for baseline consumption, questionable accuracy of the lifetime smoking history data in case-control studies, and bias in cohort studies if reducers are likelier than non-reducers to quit during follow-up. Also, the variable definitions of reduction make meta-analysis problematic. Though the results suggest some benefits of smoking reduction, more evidence is needed.