BACKGROUND AND AIMS: Smoking fewer cigarettes per day may increase the chances of stopping smoking. Capping the number of cigarettes per pack is a promising policy option, but the causal impact of such a change is unknown. This study aimed to test the hypothesis that lowering cigarette pack sizes from 25 to 20 reduces the number of cigarettes smoked.

DESIGN: This randomized controlled cross-over trial had two 14-day intervention periods with an intervening 7-day period of usual behaviour. Participants purchased their own cigarettes. They were instructed to smoke their usual brand from either one of two sizes of pack in each of two 14-day intervention periods: (a) 25 cigarettes and (b) 20 cigarettes. Participants were randomized to the order in which they smoked from the two pack sizes (a-b; b-a).

PARTICIPANTS: Participants were adult smokers in Canada who smoked from pack sizes of 25, recruited between July 2020 and June 2021. Of 252 randomized, 240 (95%) completed the study and 236 (94%) provided sufficient data for the primary analysis. The measurement was: cigarettes smoked per participant per day.

FINDINGS: Participants smoked fewer cigarettes per day from packs of 20 cigarettes than from packs of 25. After adjusting for pre-specified covariates (baseline consumption and heaviness of smoking), modelling estimated that participants smoked 1.3 fewer cigarettes per day, equivalent to 7.6% fewer from packs of 20 cigarettes.

CONCLUSIONS: Smoking from packs of 20 compared with 25 cigarettes reduced the number of cigarettes smoked per day.