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Cannabis resin – or “hash” – has increased in strength by nearly 25% over the past half century, a major international study has revealed.

Researchers with the Addiction and Mental Health Group at the University of Bath analysed 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.

Their findings, published in the journal Addiction, reveal concentrations of THC – the intoxicating component of cannabis responsible for giving users a “high” – have changed over time.

In herbal cannabis, THC concentrations increased by 14% between 1970 and 2017. This was primarily due to a rising market share of stronger varieties, such as sinsemilla. Concentrations in cannabis resin, which is extracted from herbal cannabis, increased by 24% between 1975 and 2017.

This was the equivalent of a 5mg increase in THC per annum. One 5mg dose is enough for mild intoxication, according to the researchers.

“Cannabis has continued to increase in strength over time, such that today it differs enormously from the type of drug used by people 50 years ago,” said lead author Dr Tom Freeman. “During this time attitudes have also shifted. There is now a greater appreciation of its complex interplay with mental health and potential medicinal uses.”

“Cannabis resin is often seen as a safer type of cannabis, but our findings show that it is now stronger than herbal cannabis,” said study co-author Sam Craft. “Traditionally, cannabis resin contained much lower amounts of THC with equal quantities of CBD (cannabidiol, which is believed to have some health benefits), however CBD concentrations have remained stable as THC has risen substantially, meaning it is now much more harmful than it was years ago.”

Cannabis is the most widely used illicit drug in the world but has recently been legalised in Canada, Uruguay and several states in the US.

The researchers have argued that increases in cannabis strength highlight the need for harm-reduction strategies similar to those used for alcohol – such as standard units and guidelines on safer consumption limits.

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

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