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Hands holding skunk cannabis
High-potency cannabis now dominates the UK illicit market, according to King’s College London. Photograph: King's College London/PA
High-potency cannabis now dominates the UK illicit market, according to King’s College London. Photograph: King's College London/PA

High-strength cannabis now dominates illegal market, study finds

This article is more than 6 years old

Samples showed huge rise in highly psychoactive strains, raising public health concerns

Almost all cannabis seized by police now comprises high-strength varieties, with outdoor-grown herbal strains and hashish barely found, according to a new analysis.

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.”

graphic showing rise in skunk usage
Illustration: Guardian design team

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%.

Sinsemilla cannabis analysed by King’s College London. Photograph: David Potter

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.

The researchers claim that the increasing discrepancy between THC and CBD levels in cannabis could increase the number of users at risk of developing psychotic disorders – a view supported by a number of academics not involved in the research. However, members of the medical cannabis lobby suggested it was in the interests of researchers linked to GW Pharmaceuticals to talk up the links, since the company stood to benefit from having the only legal access to cannabis-derived medicine. Three of the five authors both work for and own shares in the firm.

The use of resin has almost died out among UK cannabis users, found King’s College London researchers. Photograph: David Potter

“It’s certainly worth putting the real risk into context, which this doesn’t,” said Jon Liebling, the political director of the United Patients Alliance (UPA). “The risk of early onset psychosis due to long-term heavy use of cannabis is one in 26,000, while the risk with heavy use of street weed and starting from early age is one in 5,000.” The risk of a serious hospital admission from eating peanuts was one in 1,000, Liebling pointed out.

Clark French, also of the UPA, added: “I find that high THC cannabis aka ‘skunk’ is the only medicine that allows me function due to debilitating pain and spasms from MS. If a doctor can prescribe me this elsewhere in the world why are patients criminalised and demonised in the UK? Skunk is medicine too”

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