Campaigners pressed the Government to reclassify cannabis as new evidence emerged that the drug could more than double the risk of psychotic illnesses such as schizophrenia.
Research found any use of cannabis – which means even taking the drug just once – was associated with a 41% greater risk.
People who smoked the most cannabis were the most likely to suffer a psychotic breakdown marked by delusions, hallucinations or disordered thoughts. For frequent users, the risk rose to between 50% and 200%.
In the wake of the research, published today in The Lancet medical journal, there were calls for the Government to reverse its decision to downgrade cannabis to a class C drug.
Marjorie Wallace, chief executive of the mental health charity Sane, said: “The Lancet report justifies Sane’s campaign that downgrading a substance with such known dangers masked the mounting evidence of direct links between the use of cannabis and later psychotic illness.
“This analysis of 35 studies should act as a serious warning of the dangers of regular or heavy cannabis use, doubling the risk of developing later schizophrenia, a condition in which a person may hear voices and experience strange thoughts and paranoid delusions.”
Shadow home secretary David Davis said: “On the basis of the scientific and medical evidence available alone, the Government should reverse their disastrous policy of declassifying this harmful drug.”
The study is likely to have a big impact on the Government’s deliberations about cannabis. Prime Minister Gordon Brown is now considering whether cannabis should be returned to its previous status as a class B drug. Under Tony Blair’s premiership, the drug was downgraded from class B to class C, so possession ceased to be an arrestable offence.
Alternatively, a class B rating could be reserved just for the extra-potent form of cannabis known as “skunk”.
But Paul Corry, director of public affairs at mental health charity Rethink, said education about the dangers of cannabis was more important than reclassification.