THC Concentration in Cannabis Increased Worldwide Between 1970 and 2017

Delta-9-tetrahydrocannabinol (THC) is the major psychoactive component of cannabis. Higher potency of THC is associated with intoxication, anxiety, and cognitive impairment during use; long-term exposure has been associated with psychotic disorders and cannabis use disorder. Cannabidiol (CBD), a cannabinoid that is not psychoactive, may moderate some of these effects. Previous studies have demonstrated an increase in THC concentration in certain countries over limited time intervals. In this study, the authors conducted a systematic review of studies that reported mean THC or CBD concentrations over at least 3 annual time points.

• Twelve eligible studies were included. These collected samples from 1970 to 2017 in the US, UK, Netherlands, France, Denmark, Italy, and New Zealand.
• Ten of these studies used non-random sampling (e.g., law enforcement seizures). Otherwise, they were judged to have a low risk of bias.
• THC concentration in herbal cannabis rose annually by 0.29%.
• THC concentration in cannabis resin rose annually by 0.57%.
• CBD concentration in herbal cannabis and cannabis resin was unchanged.

Comments: Cannabis has become steadily more potent over the past 5 decades. This study quantifies these changes from an international perspective and is the first meta-analysis to describe CBD concentrations in addition to THC. Increased potency will likely lead to more adverse outcomes among people who use cannabis. Its legalization is an opportunity to regulate potency and potentially mitigate harms.

Ashish Thakrar, MD† and Darius A. Rastegar, MD

† Contributing editorial intern and addiction medicine fellow, Johns Hopkins Bayview Medical Center

Reference: Freeman TP, Craft S, Wilson J, et al. Changes in delta-9-tetrahydrocannbinol (THC) and cannabidiol (CBD) concentrations in cannabis over time: systematic review and meta-analysis. Addiction. 2021;116:1000–1010.