This indicator is based on general population surveys, which have the usual problems of substance use surveys in the last decade: non-representative sampling frame excluding key risk groups, high degree of non-response and use of self-report [56]. Moreover, the validity of prevalence estimates may vary between countries, which could increase risk of bias when comparing estimates across different countries. Undercoverage is not as easily quantified as for legal substances such as alcohol, as objective indicators such as sales data or wastewater analyses do not routinely exist or are hard to interpret in comparison to the standard indicators [57], [58], [59]. Further, the lack of uncertainty measures prohibited to apply robust statistical methods to study trends in cannabis consumption. Lastly, we do not have any information on the purpose of consumption but assume that only a minor fraction of users do take cannabis for medical purposes (for a summary of European data, see [19]). For future monitoring, survey data should distinguish between medical and recreational cannabis users.
