"As scientists, we begin every study hoping to be able to confirm beneficial effects of potential therapies, and we hoped to confirm this for the use of medical marijuana in treating Alzheimer's disease," said Dr Weihong Song, a professor of psychiatry at the University of British Columbia's Faculty of Medicine.

"But we didn't see any benefit at all."

"Instead, our study pointed to some detrimental effects."

Previous studies using rats carrying amyloid protein showed that HU210 -- a synthetic cannabinoid -- reduced the toxicity of amyloid plaques and promoted the new growth of neurons.

The latest study used mice with modified DNA containing the human genetic mutations that cause Alzheimer's -- widely considered to be a more accurate model for the disease in humans.

Mice treated with HU210 did no better than untreated mice -- those given low doses of HU210 performing the worst.

The researchers also found that HU210-treated mice had just as much plaque formation and the same density of neurons as the control group.

The mice given higher doses of HU210 overall had fewer brain cells.

Although the study showed the negative effects of cannabinoids, Dr Song believes that further studies need to be conducted.

"Our study shows that HU210 has no biological or behavioural effect on the established Alzheimer's disease model," he said.

"More studies should be done before we place much hope in marijuana's benefits for Alzheimer's patients."