Age–period–cohort analysis of trends in tobacco smoking, cannabis use, and their co‐use in the Australian population

The prevalence of tobacco smoking has declined in most high-income countries, while cannabis use has been rising. Moreover, cannabis use has been found to have increased among cigarette smokers in recent years in jurisdictions where it has been either decriminalized or legalized. This study measured trends in cannabis, tobacco and the co-use of cannabis and tobacco in Australia.Age-period-cohort analysis.Participants were n = 143 344 individuals aged 18-80 years who participated in Australia's National Drug Strategy Household Surveys (NDSHS) between 2001 and 2019.Regular (weekly/more frequently): (1) tobacco smoking only, (2) cannabis use only and (3) the co-use of cannabis and tobacco.Prevalence of only smoking tobacco decreased in all age groups (P 0.001) and birth cohorts between 2001 and 2019, but the co-use of cannabis and tobacco did not. Younger cohorts were much less likely to co-use tobacco and cannabis (P = 0.02). Period trends showed that both cannabis use only and the co-use of cannabis and tobacco have increased since 2013.There has been a consistent decrease in exclusive tobacco smoking across age, period and birth cohorts between 2001 and 2019 in Australia, although there is a recent increasing period trend in cannabis use with or without tobacco. The non-decreasing trend of co-use may reflect the strong tobacco control policies introduced over the period and changing attitudes towards cannabis use in Australia.