Estimation of the proportion of population cannabis consumption in Australia that is accounted for by daily users using Monte Carlo Simulation

Aim To estimate the proportion of cannabis consumed in Australia by daily cannabis users. Design Monte Carlo simulation using parameters estimated from nationally representative and repeated cross-sectional household surveys in 2007, 2010, 2013 and 2016. Setting Australia Participants Adult samples (mean age = 49.9; 55% females) from four National Drug Strategy Household Surveys (n = 92 243). Measurement Frequency of cannabis use (daily/weekly/about once a month/every few months/once or twice a year). The weighted estimated prevalence of users in each of these frequency levels was multiplied by population size to estimate the total number of users. Quantity of cannabis use was measured as number of joints consumed. The consumption of those who reported using bongs was converted into joints based on the bong to joint ratio estimated from the survey data. We estimated the proportion of cannabis consumed by daily users by Monte Carlo simulation using parameters estimated from the household surveys. We conducted 10 000 simulation trials, and in each trial we [1] simulated the number of users at each consumption level (stratum) based on estimated prevalence and population size[2], for each simulated individual, we simulated the number of days of cannabis use in a year based on frequency data[3], for each consumption day, we simulated the quantity consumed [4] and lastly we calculated the total joints consumed at each consumption level and estimated the proportion of joints consumed by daily users out of the total consumption. Findings The prevalence of past-year cannabis use increased from 8.9% [95% confidence interval (CI) = 8.5-9.4] in 2007 to 10.5% (95% CI = 10.0-11.1) in 2016, 16% of whom were daily users. Between 2007 and 2016, daily users accounted for between 81.6 and 85.7% of all cannabis consumed. Weekly users accounted for an additional 12.1-15.9%. Conclusion Between 2007 and 2016, only one in six Australian cannabis users were daily users, but they accounted for more than 80% of the estimated cannabis consumed in Australia.