Is today's marijuana more potent simply because it's fresher?

The average potency of illicit marijuana in the USA has increased substantially over the past four decades, and observers have suggested a number of likely reasons for this. One set of hypotheses points to a market that has evolved from foreign to domestic sources of supply, and to continuing advances in sophisticated cultivation techniques. Another set of hypotheses points to testing artifacts related to changes in the sampling, handling, and testing of illicit marijuana. The current study uses data from the federally sponsored Potency Monitoring Program, which performs ongoing forensic analysis of seized marijuana samples, to assess the extent to which the observed increase in cannabis potency in the USA between 1970 and 2010 is a function of genuine shifts in illicit marijuana markets or testing artifacts related to changes in the quality of seized marijuana. The study finds, after adjusting for marijuana quality, that the apparent 10.5 factor increase in mean reported THC% between the 1970s and the 2000s is instead on the order of a six- to seven-fold increase. By this accounting, then, the reported long-term rise in potency is roughly 57-67% as great when the quality of the tested marijuana is taken into account. This study's findings, therefore, caution against the uncritical use of potency monitoring data and highlight the importance of assessing potency measurement reliability and addressing data quality issues in future policy analytic research.