Smokers of high-strength cannabis, known as ‘skunk’, are 18 times more likely to be psychotic according to new research.
The study examined nearly 200 cannabis smokers and compared those with psychosis to those without the condition.
It strongly suggests that the use of skunk cannabis is directly linked to the development of mental illness.
The availability of skunk has soared in recent years. According to Home Office research it now accounts for between 70% and 80% of samples seized by police, compared with 15% six years ago.
The findings, from the Institute of Psychiatry at Kings College London, were announced at an annual meeting of the Royal College of Psychiatrists this week.
The strong links between cannabis use and severe mental health problems have been widely acknowledged.
The drug was controversially downgraded in 2004 to a class C status, meaning that penalties associated with it were weakened.
However, after soaring crime and a sharp rise in mental health cases the Home Secretary, Jacqui Smith, announced earlier this year that cannabis would be returned to the more serious category of class B drugs.
Richard Colwill of the mental health charity, Sane, said: “Sane has been campaigning for years that cannabis, particularly in its stronger form of skunk, can be dangerous for the significant minority of people vulnerable to mental illness.
“We have daily evidence that it can trigger frightening psychotic episodes, relapse, and in some cases a lifelong mental condition such as schizophrenia.
“It can also rob developing young minds of their potential and wreck their families’ and their own futures.”