Wednesday, 6 November, 2002, 01:33 GMT
Psychiatrists are calling for caution in the move towards licensing cannabis-based medicines.
It follows research into a possible link between cannabis use and schizophrenia.
Two recent studies have shown that heavy use of cannabis is associated with a fourfold increased risk of developing the mental illness.
"There are some dangers to using high doses of cannabis that people need to know about," said Dr Deepak Cyril D'Souza, Associate Professor of Psychiatry at Yale University School of Medicine.
He said there was concern in the medical profession that people who smoke large amounts of cannabis for a long period of time are at higher risk of developing schizophrenia.
This needs to be kept in mind in the testing of new cannabis-based medicines, he added.
He told BBC News Online: "We need to do some kind of study to establish the psychiatric safety profile of these drugs."
Dr D'Souza presented new evidence on the link between cannabis and schizophrenia at the Institute of Psychiatry at the Maudsley Hospital in London.
His research suggests cannabis may induce psychosis by its action on cannabinoid receptors in the brain.
He was speaking at the launch of the European Foundation for Psychiatry at the Maudsley.
Cliff Prior, the chief executive of Rethink, formerly the National Schizophrenia Fellowship, said: "We are looking for clear information and warnings on any prescriptions for this kind of medication, and for GPs to be made aware of the risks that may be involved.
"The use of cannabis is not thought to be a primary cause of schizophrenia, although there is strong evidence to suggest that it can trigger the onset of mental illness in some people with a prior disposition.
"More UK-based research is needed so that people with severe mental illness can have the best chance of recovering a meaningful quality of life."
Cannabis-based medicines could be available in the UK within a year following promising results in clinical trials.
GW Pharmaceuticals, the company granted a government licence to carry out tests on cannabis compounds, announced on Tuesday that advanced phase III trials had been successfully completed.
The tests, the last stage of drug evaluation before approval, showed that cannabis-based medicines can help to relieve symptoms of multiple sclerosis.
GW, which cultivates some 40,000 cannabis plants a year at a secret location in the English countryside, plans to seek marketing approval from Britain's Medicines Control Agency early next year.
Christine Jones, Chief Executive of the MS Trust, said: "We wholeheartedly agree that there is a need to investigate the psychiatric safety profile of cannabis-based medicines.
"For this reason, we are funding a study at the Institute of Neurology, London, evaluating the effects of cannabinoids on psychological factors in MS. Preliminary results will be available in September 2004.
"We are curious as to how Dr D'Souza would define 'heavy usage' and
suspect that people who use cannabinoids for the relief of MS are unlikely
to fall into this category."
© 2002, BBC