THC Content Has Increased By Up to 5.7mg Per Year Since the 1970s

Background Texture of Cannabis. Marijuana Plants Outdoor in the Home Garden
Researchers conducted random effects meta‐regression models to quantify annual changes in THC and CBD concentrations and then pooled these estimates from all international studies in random effects models.

Cannabis users who claim the drug is stronger than what they remember are probably right. According to a systematic review and meta-analysis published in Addiction, THC content has increases steadily over the past several decades.

Analyzing studies from the United States, the United Kingdom, the Netherlands, France, Denmark, Italy and New Zealand, between 1970 and 2017, the researchers found that delta‐9‐tetrahydrocannabinol (THC) content in cannabis resin has increased by .57% each year, a 28.5% increase between 1970 and 2020. Levels of cannabidiol (CBD) have remained relatively stable through the same time period. Use of high-THC cannabis is associated with an increased risk of psychotic disorders.

The researchers pulled studies from Embase, MEDLINE, Epub Ahead of Print, and In‐Process, as well as Global Health, PsycINFO and Scopus, from inception to March 2019. The researchers looked for observational studies reporting changes in THC and/or CBD concentration. Ultimately, they identified 12 studies from the US, the UK, the Netherlands, France, Denmark, Italy, and New Zealand, with studies taking place anywhere between 1970 and 2017.

THC concentrations in herbal cannabis increased by 0.29% each year (95% CI: 0.11, 0.47, P < .001) based on 66,747 cannabis samples from 8 studies conducted between 1970 and 2017.

THC concentrations in cannabis resin increased by 0.57% each year (95% CI: 0.10, 1.03, P = .017) based on 17,371 samples from 8 studies conducted between 1975 and 2017. The researchers found no evidence of changes in CBD in herbal cannabis [0.01%] (95% CI: 0.02, 0.01, P = .280) from 5 studies or cannabis resin [0.03%] (95% CI: 0.11, 0.18, P = 0.651) from 6 studies.

Limitations include the fact that nonrandom sampling methods may introduce bias. Also, the results may not be reflective of THC concentrations globally and the analysis used a small number of studies and unbalanced sample sizes.

The samples included both seized illegal cannabis and samples purchased directly from retail outlets. “Increases in THC alongside stable CBD in cannabis resin may be attributable to the inclusion of greater plant material from high THC/low CBD plants relative to traditional mixed chemotype crops,” the researchers stated. Given the increased production of high-CBD/low-THC plants, and the emerging interest in CBD’s health effects, the researchers suggest monitoring CBD for future studies.

“Concentrations of THC have continued to increase in herbal cannabis and cannabis resin,” the study authors concluded. “Increases in THC concentrations have important implications for the health effects of cannabis.”


Freeman TP, Craft S, Wilson J, et al. Changes in delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD) concentrations in cannabis over time: systematic review and meta-analysis (published online November 7, 2020). Addiction. doi: 10.1111/add.15253