Cannabis has been proven, for the first time, to be an effective treatment for the symptoms of multiple sclerosis (MS).

Many MS sufferers have taken the drug illegally, claiming that it has a therapeutic effect.

Now UK scientists have shown that a compound in cannabis can prevent muscle tremor and spasticity caused by MS.

They also found that synthetic chemicals mimicking the compound have a similar effect.

This could lead to the development of commercially-available alternatives to cannabis that have the same effect.

MS sufferers usually experience episodes of paralysis, interspersed with periods of remission.

As the disease progresses symptoms of spasticity, or muscle rigidity, and severe tremors may appear.

The researchers, from the Institute of Neurology at University College London, tested cannabis on mice who had been given an animal equivalent of MS.

The mice were injected with the cannabinoid tetrahydrocannabinol (THC) as well as three synthetic compounds.

One, methanandamide, was similar to a cannabinoid produced naturally in the body.

All had a significant ability to reduce both tremor and spasticity.

A synthetic compound called WIN55 proved the most effective against tremors, halting uncontrolled limb movements in as little as one minute.

Researcher Dr David Baker said: "The effect was really startling. It was a question of now you see the tremor, now you don't."

The compounds work by binding with two types of receptor found in the brain and spinal cord.

Blocking the receptors with other chemicals not only prevented the therapeutic action of the compounds, but made the existing symptoms much worse.

In normal circumstances, the body produces natural cannabinoids, which bind to the receptors.

The scientists believe this mechanism controls body movement, and is disrupted in MS sufferers.

They believe it could be possible to boost levels of the natural cannabinoid to improve control of body movement.

The same receptors are also found in the pain centres of the spinal cord. One of the claimed health benefits cannabis is said to have is pain relief.

Crucial role

Dr David Baker, from the Institute of Neurology, said: "Although not a cure, our research suggests that cannabinoids can play a crucial role in controlling some of the neuro-muscular problems seen with MS."

Co-author Dr Lorna Layward, from the Multiple Sclerosis Society, said: "This is the first time that we have had any objective scientific evidence that cannabinoid-like compounds have a therapeutic effect on some of the symptoms of multiple sclerosis."

She said current treatments were either not very effective or caused severe side-effects.

Tremor was particularly difficult to treat and could be severely disabling.

Dr Layward said: "I've seen people with tremor so severe that they have to lie with their arms behind their backs.

"The symptoms may be so violent that they can actually break limbs. People cannot feed themselves or get dressed by themselves. It affects their whole lives."

She said it would probably be several years before the current research could be translated into pharmaceutical products.

A Medical Research Council clinical trial is currently looking at the effect of cannabis and THC on spasticity in 600 MS patients. The trial will not produce results for about two more years.

About 85,000 people in the UK have MS. The highest incidence in the world is in Scotland, where one in 500 people have the disease. The reason is thought to be partly genetic.