Researchers at the Universities of Bristol and Cambridge wanted to see if the number of cigarettes in a single pack could help to reduce the number of cigarettes smoked, given smoking fewer cigarettes per day may increase the chances of quitting.

Packs of 20 cigarettes are standard in most countries, including the UK, although larger pack sizes are common in some countries such as Canada and Australia.

Following a study with 252 participants in Canada, researchers found that smoking from packs of 20 compared with packs of 25 cigarettes reduced the number of cigarettes smoked per day.

Participants in the study were adults who smoked from pack sizes of 25. They were asked to smoke from two sizes of cigarette packs during the study - packs that either contained 20 or 25 cigarettes.

Each participant smoked from one pack size for 14 days, then had a break, and then smoked from the other pack size for another 14 days. Participants were randomly allocated to the order in which they smoked from the two pack sizes. 

The study was presented to participants as investigating how cigarette pack size influences the effectiveness of health warnings. This was to reduce the chance of participants focusing on their cigarette consumption in relation to pack size.

Research participants smoked 1.3 fewer cigarettes per day or 7.6% fewer from packs of 20 cigarettes, compared with the packs of 25 cigarettes.

However, most of the participants preferred the larger packs, seeing them as better because they simply contained more cigarettes, they lasted longer or they were better value for money.

Smoking remains one of the largest risk factors for disease globally and is a major contributor to the gap in life expectancy and years lived in good health between the richest and poorest groups.

Professor Marcus Munafo, from the Tobacco and Alcohol Research Group at the University of Bristol, said: "This work show that we can – and should – test possible policy interventions experimentally. The findings are therefore exciting in themselves, but also reinforce the need for evidence that can support policy recommendations."

Food studies show that smaller portion and pack sizes reduce how much people eat and reducing glass and bottle sizes can reduce the amount of alcohol that people drink.

The researchers explain that smaller packs might reduce consumption because it takes effort to buy or open a new pack, or because people tend to consume a specific number of units in a pre-specified period of time.

For example, people often consume one glass or bottle of wine with dinner, and one pack of cigarettes in a day or during a two-day period. When the glass, bottle or pack contain more, more is consumed.

Professor Theresa Marteau and Ilse Lee from the University of Cambridge said: "The current study provides the first experimental evidence that people smoke more when smoking from larger pack sizes, raising the question of whether capping pack sizes might be a useful addition to existing tobacco control measures."

Biochemical measures of nicotine were not taken in this study so the team couldn’t assess the extent to which people may have compensated for smoking less cigarettes, perhaps by inhaling more or taking more puffs.

This research was conducted as part of a Wellcome Collaborative Award between the Universities of Cambridge and Bristol.

‘Cigarette pack size and consumption: a randomised cross-over trial’ by Ilse Lee, Anna K. M. Blackwell, et al. in  Addiction [open access]