Cannabis capsules failed to slow the progression of multiple sclerosis in a large British study, dealing a blow to hopes that the drug could provide long-term benefits for patients with the debilitating nerve disease.
Despite promising signs in earlier, shorter studies, researchers found patients who took capsules containing tetrahydrocannabinol (THC), a key active ingredient in cannabis, fared no better than those given a placebo.
The finding is a disappointment for researchers who thought cannabis might provide a viable therapy in the disease's secondary progressive stage, when patients have few treatment options.
Multiple sclerosis (MS) patients were assessed in the trial known as CUPID (cannabinoid use in progressive inflammatory brain disease) on both a disability scale administered by neurologists and another based on their own reporting.
"Overall the study found no evidence to support an effect of THC on MS progression in either of the main outcomes," researchers led by John Zajicek of the Peninsula College of Medicine and Dentistry, Plymouth University, said.
Results from the study, which was funded by Britain's Medical Research Council, will be presented at the Association of British Neurologists' annual meeting in Brighton later on Tuesday.
Cannabis contains more than 60 different cannabinoids, of which THC is thought to be the most active, and many MS patients have long said the drug helps them cope with the effects of the disease.
Drug companies, too, have been interested in cannabis as a medicine. Britain's GW Pharmaceuticals, working with Bayer and Almirall, recently started selling an under-the-tongue cannabis spray called Sativex to relieve spasticity.
David Nutt, professor of neuropsychopharmacology at Imperial College London, who was not involved in the latest research, said the study's failure did not mean cannabis had no role in helping MS patients.
"It would be wrong to interpret these preliminary findings to mean that cannabis does not achieve its licensed use. Cannabis is not licensed for limiting disease progression, it is licensed for dealing with spasticity and pain," he said.
Zajicek's study did find some evidence to suggest a beneficial effect in less disabled patients but because this was seen in only a small group of people it was unclear how strong the effect was.
The overall study population also experienced slower disease progression than had been expected, making it more challenging to detect any treatment effect, the research team added.
MS is a disease in which immune system cells destroy the myelin sheath that protects the nerve cells in the brain and spinal cord.
The most common type is relapsing remitting MS, affecting around 85 percent of patients at the time of diagnosis. Several drugs are available to treat this stage of the disease, including injections of beta-interferons and a new pill from Novartis called Gilenya.
Secondary progressive MS comes later and involves a sustained build up of disability.