Chemicals found in cannabis could be used to relieve symptoms of severe mental illnesses such as bipolar disorder, researchers have claimed.
The drug itself has previously been linked to an increased risk of developing such conditions.
But a University of Newcastle team, writing in the Journal of Psychopharmacology said cannabinoids might help.
Mental health campaigners called for further work to confirm this.
The Newcastle researchers said anecdotal reports from people with mental illnesses suggested cannabis could alleviate symptoms.
But they warned smoking the drug had been shown to cause long-term damage to mental health, and to increase the risk of mental illness in those who were already genetically susceptible.
Highs and lows
Scientists have been trying to find ways of harnessing the beneficial aspects of the drug without exposing people to the harmful ones.
The Newcastle team reviewed research carried out into the properties of cannabis.
They found evidence that two chemicals in cannabis could aid people with mental illness; THC (tetrahydrocannabinol) and CBD (cannabidiol).
THC helps give the 'high' associated with cannabis use, while CBD has been found to have calming properties.
Combined, they could help people with bi-polar disorder avoid the manic highs and depressed lows of their condition.
The Newcastle team say trials should now be carried out to see if the combination of chemicals does help people.
They are hoping to use a mouth spray created by GW Pharmaceuticals containing THC and CBD, which has been licensed for use for pain relief in Canada, once it is licensed in the UK.
The company is already involved in research looking at whether cannabinoids can relive pain symptoms for people with disease such as multiple sclerosis.
Heather Ashton, professor of clinical psycho pharmacology, who led the study, told the BBC News website: "If you use this mixture in the right dose and the right proportions, you might very well be able to help people with bipolar disorder, whatever way they are veering.
"We think it might be useful to patients to try, as an add-on not as a single drug, a known mixture of certain cannabinoids."
She added: "People who take cannabis for relief of these symptoms do not need the heavy doses that recreational users take."
But Professor Ashton stressed: "We all agree that smoking cannabis, especially when young, in large quantities is associated with mental illness.
"That is quite different from using it medicinally."
Jane Harris, campaigns officer at the mental health charity Rethink said: "Cannabinoids are an exciting new area for medical research, but it is important to recognise that there are over 60 active ingredients in cannabis - the two mentioned in this study may help in the treatment of bipolar disorder when taken in controlled doses.
"But for most people with severe mental illness, raw cannabis remains a risky substance.
"All medical research needs to be checked before it would make a difference to the hundreds of thousands of people living with severe mental illness in the UK."
In January this year, the government announced a review of all academic and clinical studies linking cannabis use to mental health problems.
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