Cannabis does not halt the progression of multiple sclerosis (MS), a medical trial has concluded.
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 £8m 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 he was "disappointed" the overall effect was not better.
"There's lots of evidence cannabis has a symptomatic effect - it makes people's pain, muscle stiffness and spasms better," he said.
"But what we were doing in this trial was to see if we could slow down the course of the disease.
"There are very, very few treatments for any neuro-degenerative disease, whether it's Alzheimer's, Parkinson's or progressive multiple sclerosis and we were very much hoping cannabinoid might slow down the progression of the disease as opposed to just ameliorating people's symptoms.
"I'm very disappointed - not for me - but for people with MS and I think it's desperately important that we try to find treatments that slow their progression down."
The study - involving 500 MS patients from 27 centres around the UK - was funded by the Medical Research Council (MRC) and managed by the National Institute for Health Research (NIHR) on behalf of the MRC-NIHR partnership, the Multiple Sclerosis Society and the Multiple Sclerosis Trust.
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 £5m, they would need financial backing.
"If we spent more money on these trials then we'd have answers and treatments for these degenerative diseases that we haven't got at the moment," he said.
"Progression of MS is thought to be due to death of nerve cells, and researchers around the world are desperately searching for treatments that may be 'neuroprotective'.
"Laboratory experiments have suggested that certain cannabis derivatives may be neuroprotective."