Cannabis does not help retard the progression of multiple sclerosis (MS), a recent study has revealed.
The research - the biggest study of its kind in the UK - was carried out by the Peninsula Medical School in Plymouth.
It involved patients taking pills containing the main active chemical in cannabis - tetrahydrocannabinol or THC - for three years.
The 8million-pound trial found THC did help to ease MS symptoms, but there was no evidence it slowed its progression.
Modern cannabis medications do not produce a "high" - the psychoactive ingredients are either missing or delivered in a much lower dose than in the illegal street drug.
Lead researcher, Professor John Zajicek, will present the preliminary results of the Cupid (Cannabinoid Use in Progressive Inflammatory brain Disease) trial to the Association of British Neurologists in Brighton later.
Prof Zajicek said the "holy grail" of neuroscience researchers was to try to find drugs that would actually slow the progression of neuro-degenerative diseases.
Further trials were necessary, he said, but with a cost of about 5million pound, they would need financial backing.