A chemical found in cannabis can act like an antidepressant, researchers have found.
Mental health experts warn against cannabis use
A team from Canada's University of Saskatchewan suggest the compound causes nerve cells to regenerate.
The Journal of Clinical Investigation study showed rats given a cannabinoid were less anxious and less depressed.
But UK experts warned other conflicting research had linked cannabis, and other cannabinoids, to an increased risk of depression and anxiety.
They suggested this could be because different cannabinoids acting at different levels have contradictory effects.
Cannabinoids have been shown to relieve the symptoms of multiple sclerosis and pain relief in humans.
They are naturally present in the body, as well as being found in cannabis.
The Canadian researchers gave rats injections of high levels of one artificial cannabinoid, HU210, for a month.
The animals were seen to have nerve cell regeneration in the hippocampus, which is linked to memory and emotions.
The hippocampus has been shown to generate new nerve cells throughout a person's or an animal's life, but this ability is reduced if cells are engineered to lack a cannabinoid receptor protein called CB-1.
In the Canadian study, rats given the cannabinoid were also found to be less anxious, and more willing to eat food in new environments - a change which would normally frighten them.
However, research has previously linked use of the drug cannabis to long-term damage to mental health, and to increase the risk of mental illness in those who are already genetically susceptible.
In addition, short-term high doses of cannabinoids had also been shown to produce anxiety-like effects in rats and depression-like effects in mice.
But other studies had found that low-doses of cannabinoids helped to reduce anxiety in rodents.
The Canadian team said: "These complicated effects of high and low doses of acute and chronic exposure to cannabinoids may explain the seemingly conflicting results observed in clinical studies regarding the effects of cannabinoid on anxiety and depression."
'Raw cannabis is risky'
Professor Robin Murray, of the Institute of Psychiatry, questioned whether the anti-anxiety and antidepressant effects seen in the animals would be replicated in humans.
He said: "This is a very big leap of faith as they have no data on humans, and the supposed animals' models of anxiety and depression that they use don't have much in common with the human conditions."
Paul Corry, Director of campaigns and communication at 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 - synthetic cannabinoid may be showing evidence of nerve regeneration.
"But as also pointed out in this study, the effects of cannabis on the brain are complex and produce conflicting evidence.
"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."