Background The UK (UK) became the third country in the European Union to require pictorial warnings on the back of cigarette packs, in October 2008.

Methods A repeat cross-sectional survey was conducted with 11–16-year-olds in the UK between August and September 2008 (N=1401) and August and September 2011 (N=1373). At both waves the same text warnings appeared on the front and back of packs, with the only difference being the inclusion of images on the back of packs to support the text warnings in 2011. Warning related measures assessed were salience (noticing, looking closely at warnings), depth of processing (thinking about warnings, discussing them with others), comprehension and credibility (warning comprehensibility, believability and truthfulness), unaided recall, persuasiveness (warnings as a deterrent to smoking), avoidance techniques (eg, hiding packs) and a behavioural indicator (forgoing cigarettes due to warnings).

Results For never smokers, warning persuasiveness and thinking about what warnings are telling them when the pack is in sight significantly increased from 2008 to 2011, but warning comprehensibility significantly decreased. For experimental smokers, there was a significant increase from 2008 to 2011 for warning persuasiveness, believing warnings and considering them truthful. For regular smokers, there were no significant changes from 2008 to 2011, except for an increase in hiding packs to avoid warnings and a decrease in warning salience.

Conclusions Including pictorial images on the back of cigarette packaging improved warning persuasiveness for never and experimental smokers, but had a negligible impact on regular smokers. The findings have implications for warning design.