Cannabis smoked today is ten or even 20 times stronger than when David Cameron was a teenager in the 1980s – hugely increasing the danger it poses.
The major difference is an increase in intensive indoor cultivation, rather than allowing the plant to grow naturally outdoors.
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Producers are also now using cross-fertilization techniques to produce much stronger hybrid varieties of the drug, most notably skunk.
The combination of these factors has seen the percentage the main intoxicating component within the drug increase dramatically.
In the 1970s and 1980s, cannabis commonly had a Tetrahydrocannabinol (THC) content of only one per cent.
An investigation last year found samples of the drug sold openly on the streets of London had THC concentrations of eight per cent and 11.4 per cent respectively.
Even more worryingly, the laboratory which tested the samples revealed it had examined cannabis with THC levels above 20 per cent in the past year.
Last year 55-year-old David Wyler was jailed for five years after a court heard that plants with a THC content as high as 29 per cent had been found at a house in Ilkeston, Derbyshire
As the drug’s strength has increased, scientists have increasingly linked it to mental illness.
A team from New Zealand suggested smoking cannabis virtually doubled the risk of developing schizophrenia.
It has also been linked in court to a series of killings.
Last month, former soldier David Bradley admitted the manslaughter on the grounds of diminished responsibility of four members of his family in Newcastle.
Medical experts, appearing for the prosecution and defence told the court the defendant was a heavy user of cannabis, which may have contributed to the deterioration in his mental health.
Luke Mitchell, the teenager convicted of murdering schoolgirl Jodi Jones, said he smoked the equivalent of 600 cannabis joints a week.
The High Court in Edinburgh was told that Luke Mitchell claimed his weekly intake doubled to four-and-a-half ounces at about the time Jodi died.