Back of pack picture or text warnings depicting the dangers of smoking, make little impact on teen smokers, particularly those who smoke regularly, suggests research published online in Tobacco Control.
Pictorial warnings work better than text alone, but if positioned on the back of the pack are less visible and less effective, say the researchers.
In 2008 the UK became the third European Union country to require pictorial health warnings to be carried on the back of cigarette packs.
In only five out of the 60 countries worldwide that have introduced this policy do these pictorial warnings cover more than 75% of the main surface areas of a pack, and no European country has adopted the World Health Organization standard of warnings covering half the surface area.
The researchers base their findings on the responses of more than a thousand 11 to 16 year olds in the UK, in two waves of the Youth Tobacco Policy Survey in 2008 (1401) and 2011 (1373).
The same text warnings appeared on the front and back of packs at both time points, with the only difference being the display of images on the back of packs to support the text warnings in 2011.
The teens were quizzed about the visibility and impact of the warnings; how well they served as visual cues; how easy they were to understand and believe; and how persuasive they were. Their responses were scored on a sliding scale from 1 to 5.
Most of the respondents in both waves (68-75%) had never smoked; 17-22% had experimented with cigarettes; and around one in 10 were already regular smokers, defined as smoking at least one cigarette every week.
Half the respondents in both waves said they had 'often' or 'very often' noticed the warnings, and around one in five had very often read or looked closely at them. But the percentage of regular smokers who noticed them fell from 77% in 2008 to 66% in 2011.
Overall, only one in 10 teens said they thought about the warnings when the pack was not in sight, although never smokers were significantly more likely to think about warnings 'often' or 'very often.'
At both time points, most (85%+) teens found the warnings credible, but in 2011 never smokers were less likely to find them easy to understand, while experimental smokers were more likely to find them truthful and believable.
And while the proportion of teens who thought the warnings were capable of putting them off smoking and make them less likely to smoke increased between 2008 and 2011, this only applied to never and experimental smokers. There was no change among the regular smokers.
Recall of text warnings on the pack front fell between 2008 and 2011, from 58% to 47% (Smoking Kills) and from 41% to 25% (Smoking seriously harms you and others around you), while recall of three images on the back of packs, depicting diseased lungs, rotten teeth and neck cancer, all increased.
However, recall of the other back of pack images remained below 10%, and the three text warnings on the back of packs with no supporting images were recalled by less than 1% at either time point.
The proportion of teens who said they hid the cigarette pack from others increased significantly between 2008 and 2011, but there was no increase in other avoidant behaviours. And among regular smokers, the proportion who said that the warnings stopped them from having a cigarette fell from 32% to 23%.
"As warnings need to be salient to be effective, positioning pictorial warnings only on the less visible reverse panel limits their impact," write the authors. "While recall was high at both waves for pack-front warnings, it was low (below 10%) for the pictorial warnings on the pack reverse, fear-appeal pictures aside," they add.
The fact that the UK has used the same pictures since 2008 may also have increased the "wear out" factor, particularly for regular smokers, say the authors.
"Positioning pictorial warnings only on the back of packs may have had a deterrent effect on never and experimental smokers, but for most measures no significant differences were observed. The impact on regular smokers was negligible," they conclude.
[Adolescents' response to pictorial warnings on the reverse panel of cigarette packs: a repeat cross-sectional study Online First doi 10.1136/tobacccontrol-2013-050999]