Researchers with the Addiction and Mental Health Group at the University of Bath based their findings on data from more than 80,000 cannabis street samples tested in the past 50 years in the US, UK, Netherlands, France, Denmark, Italy and New Zealand.
The team studied both herbal cannabis and cannabis resin to investigate changes in concentrations of THC – the intoxicating component of cannabis responsible for giving users a ‘high’.
Concentrations in both forms have increased over time, with lead author Dr Tom Freeman explaining that the changes in strength are so dramatic that cannabis now ‘differs enormously from the type of drug used by people 50 years ago’, The Guardian reports.
The increase was most dramatic in cannabis resin, where researchers found THC concentrations have risen 24% between 1975 and 2017.
In herbal cannabis, THC concentrations were found to have increased by 14% between 1970 and 2017. The researchers believe this is primarily due to a rising market share of stronger varieties, such as sinsemilla.
Freeman noted that as well as changes in the makeup of the drug, attitudes towards cannabis have also shifted. He explained: ‘There is now a greater appreciation of its complex interplay with mental health and potential medicinal uses.’
Study co-author Sam Craft pointed out that cannabis resin is often seen as a ‘safer’ type of cannabis, but the findings show that it is actually the stronger option.
In the study, published in the journal Addiction, the researchers argue that the increase in THC concentrations highlights the need for harm-reduction strategies regarding the drug. These could be similar to measures in place for alcohol, such as standard units and guidelines on consumption limits.