Over the past 50 years street cannabis across the world has become substantially stronger, carrying an increased risk of harm, reports the University of Bath in the United Kingdom.
The team behind the study from the Addiction and Mental Health Group at the University of Bath, synthesised data from over 80,000 cannabis samples tested in the past 50 years from street samples collected in the United States, United Kingdom, the Netherlands, France, Denmark, Italy and New Zealand.
Their findings are published in the journal Addiction and the research was funded by the Society for the Study of Addiction.
The researchers investigated how concentrations of THC (the intoxicating component of cannabis responsible for giving users a ‘high’) had changed over time in different types of cannabis.
Increased THC concentrations
In herbal cannabis, they found that THC concentrations increased by 14% from 1970 to 2017. This was primarily due to a rising market share of stronger varieties such as sinsemilla relative to traditional herbal cannabis which contains seeds and less THC.
The team have previously found consistent evidence that frequent use of cannabis with higher levels of THC carries an increased risk of problems such as addiction and psychotic disorders.
Lead author Dr Tom Freeman, Director of the Addiction and Mental Health Group at the University of Bath said: “As the strength of cannabis has increased, so too has the number of people entering treatment for cannabis use problems. More Europeans are now entering drug treatment because of cannabis than heroin or cocaine.”
The researchers found that the increases in THC were particularly high for cannabis resin, with THC concentrations rising by 24% between 1975 and 2017. Cannabis resin is extracted from herbal cannabis and is now typically stronger than herbal cannabis according to the findings.
They also looked at concentrations of cannabidiol or CBD, which is not intoxicating but may have potential medical uses such as helping people to quit cannabis. In contrast to THC, they found no evidence for changes in CBD in cannabis over time.
Study co-author Sam Craft also from the Addiction and Mental Health Group at the University of Bath explained: “Cannabis resin – or ‘hash’ – is often seen as a safer type of cannabis, but our findings show that it is now stronger than herbal cannabis.
Traditionally, cannabis resin contained much lower amounts of THC with equal quantities of CBD, however CBD concentrations have remained stable as THC has risen substantially, meaning it is now much more harmful than it was many years ago.”
Relevance of findings
Cannabis is the most widely used illicit drug in the world but has recently been legalised in Canada, Uruguay and several states in the United States.
The findings of this new study have particular relevance in light of growing demands to legalise cannabis in an attempt to make it safer. Most recently a referendum in New Zealand (which ultimately failed to receive public support) included measures to limit the strength of cannabis sold through legalisation.
Standard units for safer consumption?
The researchers argue that increases in cannabis strength highlight the need to implement wider strategies for harm reduction similar to those used for alcohol – such as standard units and public guidelines on safer consumption limits.
Dr Tom Freeman added: “As the strength of cannabis has risen, consumers are faced with limited information to help them monitor their intake and guide decisions about relative benefits and risks.
The introduction of a standard unit system for cannabis – similar to standard alcohol units – could help people to limit their consumption and use it more safely.”
Changes in delta‐9‐tetrahydrocannabinol (THC) and cannabidiol (CBD) concentrations in cannabis over time: systematic review and meta‐analysis
Addiction. Published on 7 November 2020
Authors
Tom P Freeman, Sam Craft, Jack Wilson, Stephan Stylianou, Mahmoud El Sohly, Marta Di Forti and Michael T Lynskey
Abstract
Cannabis products with high delta‐9‐tetrahydrocannabinol (THC) concentrations carry an increased risk of addiction and mental health disorders, while it has been suggested that cannabidiol (CBD) may moderate the effects of THC.
This study aimed to systematically review and meta‐analyse changes in THC and CBD concentrations in cannabis over time (PROSPERO registration: CRD42019130055).
Design
Embase, MEDLINE® and Epub Ahead of Print, In‐Process and Other Non‐Indexed Citations and Daily, Global Health, PsycINFO and Scopus were searched from inception to 27-03-2019 for observational studies reporting changes in mean THC and/or CBD concentration in cannabis over at least three annual time points.
Searches and extraction were conducted by two independent reviewers. Random effects meta‐regression models estimated annual changes in THC and CBD for each product within each study; these estimates were pooled across studies in random effects models.
Results
We identified 12 eligible studies from the USA, UK, Netherlands, France, Denmark, Italy and New Zealand. For all herbal cannabis, THC concentrations increased by 0.29% each year (95% CI: 0.11, 0.47), P < 0.001 based on 66 747 cannabis samples from eight studies, 1970–2017.
For cannabis resin, THC concentrations increased by 0.57% each year (95% CI: 0.10, 1.03), P = 0.017 based on 17 371 samples from eight studies, 1975–2017. There was no evidence for changes in CBD in herbal cannabis [−0.01% (95% CI: −0.02, 0.01), P = 0.280; 49 434 samples from five studies, 1995–2017] or cannabis resin [0.03% (95% CI: −0.11, 0.18), P = 0.651; 11 382 samples from six studies, 1992–2017].
Risk of bias was low apart from non‐random sampling in most studies. There was evidence of moderate to substantial heterogeneity.
Conclusions
Concentrations of delta‐9‐tetrahydrocannabinol (THC) in international cannabis markets increased from 1970 to 2017 while cannabidiol (CBD) remained stable.
Increases in THC were greater in cannabis resin than herbal cannabis. Rising THC in herbal cannabis was attributable to an increased market share of high‐THC sinsemilla relative to low‐THC traditional herbal cannabis.
[link url="https://www.bath.ac.uk/announcements/cannabis-strength-soars-over-past-half-century-new-study/"]University of Bath material: Cannabis strength soars over past half century – new study[/link]
[link url="https://onlinelibrary.wiley.com/doi/10.1111/add.15253"]Article in the journal Addiction – Changes in delta‐9‐tetrahydrocannabinol (THC) and cannabidiol (CBD) concentrations in cannabis over time: systematic review and meta‐analysis[/link]