Cannabis or marijuana does not slow the progression of multiple sclerosis (MS), a large medical trial has concluded.
Despite promising results from earlier, shorter studies, UK researchers at the Peninsula Medical School in Plymouth found that patients who were given cannabis capsules containing tetrahydrocannabinol (THC), a main active ingredient in marijuana, did not have better results than those given a placebo.
The latest findings are a disappointment for researchers who thought that the drug could provide effective therapy for patients in the disease's secondary progressive stage and who have few treatment options.
The study consisted of 493 people with progressive MS and took eight years to complete. Participants were asked to take either THC or placebo capsules for three years.
In the clinical trial known as CUPID (cannabinoid use in progressive inflammatory brain disease), multiple sclerosis patients (MS) were evaluated on both a disability scale administered by neurologists and another based on their own reporting.
However, while overall the study found no evidence to support an effect of THC on MS progression, there was some evidence to suggest a beneficial effect in participants at the lower levels of disability at the start of the study, but additional studies that include more participants would be needed to confirm this.
"Overall the study found no evidence to support an effect of THC on MS progression in either of the main outcomes," lead researcher John Zajicek wrote in the study.
The study, which was funded by the Britain's Medical Research Council, will be presented at the Association of British Neurologists' annual meeting in Brighton on Tuesday.
Cannabis contains more than 60 different cannabinoids, and THC is believed to be the most active, and many MS patients have reported that the drug helps them cope with the symptoms of the disease.
Many pharmaceutical companies have been interested in medicinal uses of marijuana, and UK's GW Pharmaceuticals, partnering with Bayer and Almirall, had recently started selling an under-the-tongue cannabis spray called Sativex to relieve spasticity, or muscle tightness related to multiple sclerosis.
Professor David Nutt, who was not involved in the latest research and teaches neuropsychopharmacology at Imperial College London, said that the study's failure did not mean cannabis was no help to 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, according to Reuters.
Multiple sclerosis is a disease in which cells in the immune system destroy the myelin sheath that protects the nerve cells in the brain and spinal cord, and the most common type is lapsing remitting MS, which affects around 85 percent of patients at the time of diagnosis.
Secondary progressive MS comes after some time and involves a persistent accumulation of disease symptoms and disability.