Background Tobacco industry documents illustrate how packaging promotes smoking experimentation and reinforces existing smokers' behaviour. Plain packaging reduces the perceived attractiveness of smoking and creates an opportunity to introduce larger pictorial warnings that could promote cessation-linked behaviours. However, little is known about the effects such a combined policy measure would have on smokers' behaviour.

Methods A 3 (warning size) *4 (branding level) plus control (completely plain pack) best-worst experiment was conducted via face-to-face interviews with 292 young adult smokers from a New Zealand provincial city. The Juster Scale was also used to estimate cessation-linked behaviours among participants.

Results Of the 13 options tested, respondents were significantly less likely to choose those featuring fewer branding elements or larger health warnings. Options that featured more branding elements were still preferred even when they also featured a 50% health warning, but were significantly less likely to be chosen when they featured a 75% warning. Comparison of a control pack representing the status quo (branded with 30% front of pack warning) and a plain pack (with a 75% warning) revealed the latter would be significantly more likely to elicit cessation-related behaviours.

Conclusions Plain packs that feature large graphic health warnings are significantly more likely to promote cessation among young adult smokers than fully or partially branded packs. The findings support the introduction of plain packaging and suggest use of unbranded package space to feature larger health warnings would further promote cessation.