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Cannabis Resin and Flower Are Much More Potent Than Fifty Years Ago, Study Finds

By Alexander Beadle

Published: Nov 18, 2020   
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A new study examining global trends in THC and CBD concentration in illicit cannabis has found a substantial increase in the potency of the products since 1970, raising concerns about an increased potential for harm and misuse.

The study, led by researchers from the Addiction and Mental Health Group at the University of Bath, found notable increases in the strength of both cannabis flower products and cannabis resin (also known as “hash”); THC levels present in the resin have risen by nearly 25 percent over the past 50 years.

Given the increasing potency of these products, the research team has stressed the need to implement effective harm reduction strategies so that consumers can make safer and more informed choices about their cannabis use.

Cannabis resin THC concentration grows by 0.57 percent annually

In total, the researchers analyzed data from over 80,000 cannabis samples reported in the medical literature between 1970 and 2017, covering studies from the USA, UK, Netherlands, France, Denmark, Italy, and New Zealand. The vast majority of the studies dealt with illicit cannabis material that had been seized, although two Dutch studies did also include information on randomized samples obtained from retail shops.

Within the herbal cannabis samples reported, they found that THC concentrations had risen by 14 percent across the last half-century, primarily due to the rising market share of more potent varieties of cannabis.

For cannabis resin, the observed increase in THC was significantly higher, a rise of 24 percent between 1975 and 2017. This suggests that the quantity of THC found in a typical gram of cannabis resin is rising by around 5.7 milligrams each year – an amount that could cause mild intoxication even by itself, the researchers say.

“Cannabis has continued to increase in strength over time, such that today it differs enormously from the type of drug used by people fifty years ago,” lead author Dr Tom Freeman told the Guardian. “During this time attitudes have also shifted. There is now a greater appreciation of its complex interplay with mental health and potential medicinal uses.”

Indeed, recent studies on high-CBD cannabis products have found them effective in reducing the symptoms of obsessive-compulsive disorder, and even protecting against the long-term psychiatric risks sometimes associated with high-THC products.

“Cannabis resin is often seen as a safer type of cannabis, but our findings show that it is now stronger than herbal cannabis,” added study co-author Sam Craft in a statement. “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."

What does this mean for public health?

The University of Bath research team is well aware of the potential risks that can come along with long-term exposure to high levels of THC. Freeman has also previously been part of a team that authored a review in Nature Reviews Neuroscience on cannabis and its effects on cognition and addiction.

“As the strength of cannabis has increased, so too has the number of people entering treatment for cannabis use problems,” Freeman explained. “More Europeans are now entering drug treatment because of cannabis than heroin or cocaine.”

This issue of cannabis misuse and dependence is of particular importance today as more countries consider legalizing the drug’s use. In some cases, this pressure to allow legalization is even directly linked to the belief that legalization will allow for better harm reduction strategies. For example, in a discussion paper authored before Canada officially legalized cannabis for recreational use, the Canadian Nurses Association noted that legalization could allow for “the regulation of quality, dose, and potency” of cannabis by the government, as well as potentially being an avenue to “minimize social harms and eliminate the costs of prohibition[…] which could then be used for prevention, education, health, and social programs.”

The researchers behind the new potency study argue that the rising strength of cannabis products further highlights this need for wider harm reduction strategies. Specifically, they say cannabis should be treated like alcohol.

“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.”

The University of Bath researchers previously suggested the idea of introducing alcohol-style consumption units for cannabis in a 2019 paper published in Addiction. After reviewing experimental and ecological data, public health considerations, and existing policy, the team proposed the introduction of a “standard THC unit” fixed at five milligrams THC for all forms of cannabis products.


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