Commentary on Freeman et al. (2019): Why does cannabis potency still vary across European countries?

Abstract

Although the potency of cannabis has increased in almost every European country between 2006 and 2016, substantial differences among countries persist, with important public health consequences. Possible explanations for variations across Europe in the cannabis market include tastes (demand-side) and enforcement or input price differences (supply-side). Freeman et al. 4 contribute new observations to this puzzle of varying equilibria in national markets. They present data on the potency of cannabis [as measured by the percentage of tetrahydrocannabinol (THC)] in 21 European countries during the period 2006–16. For all but two countries, potency of herbal cannabis increases during that period, mostly by a substantial amount. There is reason to believe that there are common factors at work here, and Freeman and collaborators suggest that there has been an improvement in technology for cannabis production. However, in 2016, visual inspection of the fitted graphs for herbal cannabis (4, Fig. S2) suggests a range of average potencies almost as broad as in 2006. Putting aside the Netherlands, with its quasi-legal market for cannabis and unchanging potency of 15%, the 2016 range is approximately 3–15%, whereas in 2006 it was approximately 0.5–10%. Similar patterns can be observed for cannabis resin (4, Fig. S1). There is little evidence of a convergence or harmonizing across markets. As with purity in the cocaine and heroin markets, cannabis potency is a product decision by producers, which partially reflects customer taste. The variation in potency probably matters for public health, as Freeman et al. note, because user titration is imperfect; those who consume very high-potency cannabis may consume more THC. Thus, it would be useful to investigate what factors drive potency variation. To my knowledge, no such effort has been published. It is not necessary to rely on national-level data for such research. There may well be similar variation among cities within countries that would allow exploration of these hypotheses where city-level data are available. The fact that almost all European nations have experienced an increase in potency does not lessen the significance of the national variation; it merely deepens the puzzle. None.