By Paul Rincon
BBC News Online science staff, at the BA festival
The biggest UK study of cannabis-based drugs has shown evidence for a long-term benefit in easing the symptoms of multiple sclerosis (MS).
The drugs contain active compounds extracted from the plant
"There is some evidence of a long-term effect," Dr John Zajicek, who heads the trial, confirmed to the BA Festival of Science at Exeter University.
He also said the data so far "were consistent" with the idea the drugs could arrest nerve death in sufferers.
He was presenting results that update a study published in The Lancet last year.
This 15-week research project revealed patients using cannabinoid compounds could find relief from some of the painful symptoms of MS.
But an analysis of the data suggested there was little reduction in spasticity among the research subjects - one of the key tests used to assess the drugs.
Spasticity refers to feelings of stiffness and a wide range of involuntary muscle spasms or sudden movements.
Speaking at the annual British Association meeting, Dr Zajicek, from the Peninsula Medical School in Devon, said this assessment might have been premature.
Longer-term monitoring, he said, had now shown patients experiencing significant improvement in this area.
In MS, the protective sheath that surrounds nerves (myelin) wastes away, leaving scar tissue known as a sclerosis. Sometimes the nerve fibre itself is damaged.
This disrupts the ability of the nerves to conduct electrical impulses to and from the brain, producing the various symptoms of MS. These include pain, deadening fatigue, problems with sight, mobility and coordination.
"We have generated some very interesting results which suggest there may be a potential long-term benefit from these drugs," Dr Zaijcek told journalists.
"There was evidence of an effect on spasticity and on disability."
Asked whether cannabis might work by arresting the death of nerve cells in
MS sufferers, Dr Zajicek answered: "Our results are certainly consistent with that hypothesis."
Patients were given up to 25mg per day of either whole cannabis extract in pill form, a tetrahydrocannabinol (THC) pill, or a placebo. The first trial, with 667 volunteers, lasted 15 weeks and was published in The Lancet in November.
These subjects were then asked if they wanted to participate in a further
52-week trial. About two-thirds of the original participants signed up for this long-term study.
A strong beneficial effect on symptoms of the disease was reported by patients throughout the 15-week trial and the one-year study. But this did not by itself demonstrate an underlying physical basis for this effect, Dr Zajicek admitted.
"What we've been trying to do is to have some objective, independent evidence of that," said Dr Zajicek.
Test of safety
Asked whether he thought cannabis should be approved as a medicine, Dr Zajicek answered:
"I think the licensing agencies need to assess all the evidence and we need further, long-term studies before we make that decision."
"This comes with lots of caveats. The experiment was only designed for a 15-week trial. We followed the subjects up expecting to find out only whether cannabis was safe in the longer term.
"But some of the experimental data emerged over the course of our study."
In the last 15 years, scientists have discovered a variety of cannabis-like
chemicals (or cannabinoids) in the brain.
They reduce the amount of neurotransmitters that mediate communication between nerve cells.
Certain cannabinoids may have more of an effect than others, Dr Zajicek said.
These cannabis-like chemicals have been used as treatments for increasing appetite associated with cancer and Aids, as well as various movement disorders.