A new study has found THC concentrations in “herbal cannabis” and “cannabis resin” — more commonly referred to as “hash” — have risen significantly over the past 50 years.
Conducted by the Addiction and Mental Health Group at the University of Bath, The study analyzed more than 80,000 samples from the United States, the United Kingdom, the Netherlands, France, Denmark, Italy and New Zealand. The researchers found that from 1970 to 2017, the concentration of THC in cannabis increased by 0.29% annually, an overall increase of about 14%. At the same time, the concentration of THC in hash products increased by 0.57% annually from 1975 to 2017, an increase of about 24%.
Notably, an analysis of cannabis flower and hash products that were seized between 1995 and 2017 found no increase in CBD concentrations.
“In the context of typical use, our findings suggest that the quantity of THC in a typical gram of cannabis rose by 2.9 milligrams each year for all herbal cannabis, and by 5.7 milligrams each year for cannabis resin. These annual increases in milligrams of THC per gram of cannabis are in the range of low single doses that can produce mild intoxication, similar to a ‘Standard THC Unit’ of 5 milligrams. Changes in THC concentrations over time could also influence the efficacy and safety of cannabis used for medicinal purposes in the absence of standardized dosing information for illicit cannabis products.” — Excerpt from the study
The researchers believe the increase of THC in cannabis flower was due to an increase of the market share for sinsemilla — or cannabis with high concentrations of THC — and not due to an overall increase in THC among specific cultivars. Furthermore, they believe the elevation of THC concentrations in hash products (while CBD concentrations remained steady) can be explained by the increase of THC-rich material at the point of cannabis resin production.
Despite considering studies from around the world, the authors point out that since most of the studies included in their studies are from the United States, their results are not "globally representative". In addition, "non random" sampling by law enforcement agencies may lead to potential bias in the study.