Abstract
INTRODUCTION: Adding messages to cigarette health warning labels (HWLs) about the harms of smoking increases awareness of these health facts, but little is known about the impact of removing messages. This is the first study to directly investigate the impact of adding and removing messages from cigarette HWLs on smokers' awareness of harms. METHODS: Data were drawn from nine waves of the International Tobacco Control (ITC) Canada Survey, a national representative cohort of adult smokers (n=5863) conducted nearly annually between 2002 and 2013-2014. Two analytical approaches were conducted: generalised estimating equation (GEE) regression models estimated adjusted percentages of correct smoking-related health statements at each wave and segmented regression analyses modelled temporal trends in awareness before and after the revisions by measuring the difference in slopes. RESULTS: Adding messages to HWLs significantly increased awareness that smoking causes blindness (OR=3.36 (95% CI 2.71 to 4.18); p<0.001; estimated increase of 1.01 million smokers in Canada) and bladder cancer (OR=2.14 (95% CI 1.71 to 2.66), p<0.001; estimated increase of 1.09 million smokers). Adding the warning that nicotine causes addiction did not significantly impact smokers' awareness. Removing messages was shown to decrease awareness that cigarette smoke contains carbon monoxide (OR=0.53 (95% CI 0.41 to 0.70), p<0.001; estimated decrease of 342 000 smokers) and smoking causes impotence (p=0.007 for the difference in slopes; estimated decrease of 354 000 smokers). CONCLUSIONS: Adding messages to HWLs increases smokers' awareness of health facts, but removing messages decreases awareness. These findings demonstrate the importance of carefully considering the implications of adding and especially removing messages from HWLs and the importance of regularly revising warnings.