In the first study of its kind for 10 years, researchers from GW Pharmaceuticals, which makes medicines derived from cannabis, and King’s College London found 94% of samples seized by police were varieties with a high psychoactive content, suggesting they dominate the illicit market. The researchers warned of “concerning implications for public health”.

“We are talking about a market where pretty much all the samples that the police come across are represented by this high-potency [product],” said Dr Marta Di Forti, one of the authors from King’s College London. “This is actually worrying because if we do believe that there are types of cannabis that can be used more safely – because of the presence of cannabidiol – well, at least in this country, these are disappearing.”

The research team defined high-potency strains as those with an average THC content of 14%, and practically no detectable traces of CBD (cannabidiol). Of the compounds in cannabis, THC (tetrahydrocannabinol) gives users a high, but can impair attention, concentration and memory, and produce symptoms of psychosis. CBD has been shown to reduce symptoms of psychosis and ameliorate impairments caused by THC, but has no psychoactive effect.

The last time comparable research was carried out in 2005, high-potency strains accounted for 51% of seizures. A wider study carried out by the Home Office in 2008 found they accounted for 85%.

For the latest research Di Forti and her colleagues asked police in Derbyshire, Kent, Merseyside, Sussex and London – the same forces researched in 2005 – to supply seized cannabis samples. From each, they analysed a 250mg subsample – “a typically generous weight of cannabis used in a single joint,” according to their paper, published in Drug Testing and Analysis.

Despite the predominance of high-strength strains, results showed there had been little increase in the average strength. However, hashish (resin) samples, which previously had 1:1 ratios of THC and CBD, about 4% of each, had an average 6% THC and 2% CBD, a ratio of 3:1. The most potent samples discovered were trichome powders, often home-made by users in special grinders with a mesh to sieve off the trichomes from the buds, the strongest of which was 46% THC with 0.2% CBD.

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