Experts are warning of potential mental health risks after they found most of the cannabis consumed in the UK is super-strength sinsemilla – or skunk.
It comes after they found highly-potent strains of cannabis dominated the illegal market in Britain after researchers discovered they accounted for the vast majority of police seizures in a sample.
The proportion of confiscations rose from just over a half in 2005 to 94% in 2016, according to the study.
Dr Marta Di Forti, Medical Research Council clinician scientist at King's College London's Institute of Psychiatry, Psychology & Neuroscience, said: "In previous research we have shown that regular users of high-potency cannabis carry the highest risk for psychotic disorders, compared to those who have never used cannabis.
"The increase of high-potency cannabis on the streets poses a significant hazard to users' mental health, and reduces their ability to choose more benign types."
The researchers looked at almost a 1000 seizures from London, Kent, Derbyshire, Merseyside and Sussex – the same areas were sampled in 2005 and 2008.
They found 94% of cannabis seizures in 2016 were for skunk, compared to 85% in 2008 and 51% in 2005.
The increase was put down to a reduction in the availability of weaker cannabis resin – from 43% in 2005 and 14% in 2008 to just 6% in 2016.
The study, published in the Drug Testing and Analysis journal, found the average concentration of THC – the primary psychoactive component of cannabis – remained at 14% for the sinsemilla strain between 2005 and 2016.
However, the average THC level in resin went up from 4% to 6% in the same period.
"More attention, effort and funding should be given to public education on the different types of street cannabis and their potential hazards," said Dr Di Forti.
"Public education is the most powerful tool to succeed in primary prevention, as the work done on tobacco use has proven."
Cannabis is now the most commonly used drug according to findings for England and Wales, with 6.6% of people aged between 16 and 59 – 2.2 million people – using it in the past year.
More from Cannabis
However, figures show long-term cannabis use is on the decline and there has been a fall in the number of possession of cannabis offences recorded by police.
Police numbers for last year show offences are down by almost a tenth.
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