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Centre for Behavioural
Research in Cancer

Effect of pack size on cigarette consumption

Observational evidence suggests that cigarette pack size – the number of cigarettes in a single pack – is associated with consumption but experimental evidence of a causal relationship is lacking. We conducted a two-stage adaptive parallel group randomised controlled trial in which Australian smokers who usually purchased packs containing at least 25 cigarettes were randomised to use only packs containing either 20 (intervention) or their usual packs (control) for four weeks. The primary outcome, the average number of cigarettes smoked per day, was measured through collecting all finished cigarette packs, labelled with the number of cigarettes participants smoked. An interim sample size estimation was used to evaluate the possibility of detecting a meaningful difference in the primary outcome. The interim analysis, conducted when 124 participants had been randomised, suggested 1122 additional participants needed to be randomised for sufficient power to detect a meaningful effect.  This exceeded pre-specified criteria for feasible recruitment and data collection was terminated accordingly. Analysis of complete data (n=79) indicated that the mean cigarettes smoked per day was 15.9 (SD=8.5) in the intervention arm and 16.8 (SD=6.7) among controls (difference 0.9; 95% CI = -4.3, 2.6).  The results of this study were unclear as to whether reducing pack sizes from at least 25 to 20 reduces consumption, although the results provided no evidence that capping cigarette pack sizes would be ineffective at reducing smoking. Learnings from this Australian preliminary study informed the development of a more efficient randomised controlled trial later conducted in Canada (, which provided strong evidence that reducing pack sizes from 25 to 20 cigarettes reduced the number of cigarettes smoked per day.  

CBRC staff

Prof Melanie Wakefield, Dr Michelle Scollo, Megan Bayly


Prof Theresa Marteau (Cambridge University) and UK colleagues


Wellcome Trust, via Cambridge University


2018 - 2021