Commentary on Niesink et al. (2015): Interpreting trends in tetrahydrocannabinol potency—three stories, one of which may be true

The iron law of prohibition. Almost 30 years ago, Richard Cowan [3] argued that ‘the iron law of drug prohibition is that the more intense the law enforcement, the more potent the drugs will become’, a thesis developed further by Mark Thornton [4]. The Dutch case (and arguably the recent US case as well) pretty thoroughly demolishes this hypothesis. If anything, the lenient Dutch experienced greater increases in potency than their tougher neighbors, and now potency seems to have leveled off at a time when, if anything, Dutch policies are becoming tougher.

The Sword of Damocles. I have been telling a variant of this story for some years now. The idea is that the brilliance of the ambiguous Dutch policy is that it affords a degree of government control over the industry that is difficult to achieve in either a fully prohibited or fully legalized market. Dutch officials began floating proposals to cap tolerated products at 15% as early as 2011. Although legislation has not yet been passed, cannabis proprietors surely understand that their situation is perilous, especially given the recent curtailment of tourist sales and the closing of a quarter of the shops. This leads to a testable prediction: we should see a discontinuity in the data once the threat of the 15% ceiling was raised, with post-2011 samples piling up just below the 15% threshold. Because the authors shared their data with me I was able to test this in various ways, but the story did not hold up (see Fig. 1). Unexpectedly, there is actually a greater positive skew in the data after 2011 (1.89) than before 2011 (1.77), despite the mean decline.

Learning by doing. The third story is that both producers [5] and consumers [6] ‘learn by doing’, producing changes in their behavior over time. First, potency rose over the years because of what unquestionably have been major technical advances in cannabis horticulture, and the recent downturn could indicate that Dutch users are deciding that highly potent cannabis has drawbacks that outweigh its advantages. This story finds some support in research by Korf and colleagues [7], who find that a subtype of Dutch users who prefer the strongest cannabis tend to be younger, and to moderate their use as they age. This is a story that seems to fit the data and it is a story I would like to believe, given the correlation between US potency rates and indicators such as treatment admissions, emergency-room incidents and the rising frequency of daily use among cannabis users. However, this story also conflicts with some facts. Users have already had many generations to learn about cannabis, and indeed one very potent product—hash oil—is recently surging in popularity [8], despite having been available in the Netherlands and other countries for many decades.