Almost all cannabis sold on British streets can cause psychosis after weaker forms were driven from the market.

The most potent “skunk” accounts for 94 per cent of all cannabis seized by police, up from half in 2005, according to the first study for almost a decade.

Dealers are thought to be pushing higher-strength products to get recreational users hooked, with the milder hashish form barely available, researchers say.

Teenager cannabis smokers have been told that skunk is more dangerous and that they must watch out for paranoia and other symptoms of psychosis.

Skunk, also known as sinsemilla, is made from unpollinated cannabis and contains higher levels of THC, a psychoactive compound, than herbal marijuana or resin, also known as hashish.

A Home Office study of police seizures in 2005 found that 51 per cent were skunk and 43 per cent resin. Three years later skunk seemed to be becoming stronger and more common, but the study has not been repeated since 2008.

Now researchers at King’s College London have analysed almost 1,000 samples seized by police in London, Merseyside, Derbyshire, Kent and Sussex. Resin accounted for just 6 per cent of samples, falling to 3 per cent in London, and even that had become stronger since 2008, according to results published in Drug Testing and Analysis.

“The increase of high-potency cannabis on the streets poses a significant hazard to users’ mental health,” said Marta Di Forti, senior author of the paper. “It’s a big worry. It’s pretty much the only kind of cannabis you can buy out there.”

Her previous work suggests that skunk users are five times more likely to develop psychosis than non-users, while there is no extra risk for hash smokers. Britain is largely self-sufficient in skunk as farms take over from hash grown in Morocco and Dr Di Forti said that the stronger product could be a deliberate policy by gangs.

“If high potency is more likely to induce dependence, that’s an advantage for the drug dealer because he wants people to come back as much as possible, rather than recreational users who only use at the weekend when they’re listening to music or going to a party,” she said.

Skunk has not got stronger since 2005, which she said could be because users could not tolerate higher THC concentrations without side-effects.

About 2.2 million people are estimated to have smoked cannabis in the past year, a million of them aged 16-24.

While there is some evidence that users can partially detect higher strength cannabis and cut back, Ian Hamilton, a lecturer in mental health at the University of York, said: “If the cannabis market is saturated with higher potency cannabis this increases the risk of younger and more naive users developing problems as they are less likely to adjust the amount of cannabis they ingest than more experienced users.”

Source: https://www.thetimes.co.uk/edition/news/mental-illness-risk-as-skunk-drives-out-milder-cannabis-wgd58b56l# February 2018