A Birmingham psychology expert last night said the results of studies into the side-effects of cannabis smoking were still unclear, as the Home Secretary rejected calls to reverse the Government's decision to downgrade the drug.
Dr Nicholas Barnes, a reader in neuropharmacology at the University of Birmingham, said mental health experts used "the numbers game" in proving that clinical depression and schizophrenia were linked to cannabis use.
However, he said he believed the drug should continue to be illegal and treated as a "substance of abuse".
The Home Secretary Charles Clarke yesterday refused to reclassify cannabis - which was downgraded as a Class C drug - following the emergence of an unpublished report.
The Advisory Council on the Misuse of Drugs apparently found the impact of smoking cannabis on mental health was more serious than previously thought.
The council is said to have stopped short of recommending reclassification and many drugs experts believe that would be counter-productive.
But Mr Clarke did disclose plans for a radical overhaul of the way drugs are categorised and prohibited, which dates back to the 1971 Misuse of Drugs Act.
Dr Barnes said: "The studies into cannabis are usually a numbers game. You cannot say if you take cannabis you will get clinical depression - you just look at the numbers. It is not black and white.
"If you are a cannabis addict there is a greater risk, but it is the same argument that the tobacco industry has used. You cannot prove that there is a link between lung cancer and smoking, but more people who smoke get lung cancer.
"We do not actually know how cannabis smoking leads to depression. We do not even understand the neurochemistry of depression."
Dr Barnes refused to be drawn into the political row developing over the classification of cannabis. However, he said personally he opposed any legalisation of the drug.
He said: "My personal opinion - not my professional opinion - is that it is a drug of abuse and it should not be legalised. It should be classified as a drug of abuse.
"If people take cannabis they are breaking the law and that is the way it should be."
Dr Barnes said he would only accept the legalisation of the drug if it was in the form of a medicine that patients would be prescribed from a local hospital.
He said it could be prescribed as a pill, such as nabilone, which is chemically related to marijuana.
Concerns about a link between super-strength cannabis varieties and mental illness have mounted since Mr Clarke's predecessor David Blunkett downgraded the drug in January 2004, making cannabis possession a nonarrestable offence in most cases.
Mr Clarke said he would launch a major public information campaign to stress that cannabis was "anything but harmless".
"Everyone needs to under-stand that cannabis is harmful and it is illegal," he said. "Our education and health campaigns will clearly transmit that message."
Mr Clarke said he was accepting the advice of the ACMD to keep cannabis as a Class C drug.
Shadow Home Secretary David Davis criticised Mr Clarke's "tragic" decision and accused him of failing to "grasp the nettle".
Mr Davis said: "I am frankly disappointed that you have not decided to grasp the nettle and reclassify cannabis back to a Class B drug." ..SUPL: