The effect of occasional smoking on smoking-related cancers: in the European Prospective Investigation into Cancer and Nutrition (EPIC).

Objective: Most studies on tobacco smoking have focused on daily-smokers. Occasional smokers, who have never smoked daily, have often been included in the reference group of never-smokers. We have investigated the association between occasional smoking and cancer of the bladder, kidney, pancreas, upper aero-digestive tract and lung.
Methods: The study population consisted of 158,488 persons, who provided information on occasional smoking, within the European Prospective Investigation into Cancer and Nutrition (EPIC), 780 of whom developed a smoking-related cancer. We used Cox proportional hazard model, stratified by gender and country to estimate incidence rate ratios (IRR) for smoking-related cancers.
Results: The results suggest that occasional smokers have a higher risk of bladder cancer (IRR: 1.92, 95% confidence interval (CI) 0.93-3.98) and of the major smoking-related cancers combined (IRR: 1.24, 95% CI 0.80-1.94) than true never-smokers. Including occasional smokers in the reference group resulted in a lower risk estimate for former and current smokers.
Conclusions: Occasional smoking should be discouraged.