A cannabis drug offers new hope to thousands with untreatable forms of epilepsy, a groundbreaking study has shown.

The cannabidiol (CBD) compound reduced convulsive seizures by nearly 40 percent in teenagers and children with Dravet syndrome, a rare and severe epileptic condition that does not respond to traditional drug treatments.

In five percent of cases the debilitating fits that afflicted some patients every day stopped altogether.

The drug's British manufacturers hope their product will be licensed for treating Dravet syndrome in the UK by the end of the year.

But its full potential could be much bigger if future research shows it can control the symptoms of other forms of epilepsy in both children and adults.

Around one in 200 of the UK population suffers from epilepsy, a wide-ranging disorder with more than 130 known causes. A third of patients are not helped by existing anti-seizure drugs such as sodium valproate and carbamazepine.

For those families for whom convulsive seizures have been reduced, they are able to go out more; the children are more awake, in their opinion, and more interactive.

The international trial's co-leader from New York University Langone Medical Center, Professor Orrin Devinsky, said: "Cannabidiol should not be viewed as a panacea for epilepsy, but for patients with especially severe forms who have not responded to numerous medications, these results provide hope that we may soon have another treatment option."

CBD is a medical-grade cannabis extract containing virtually no high-inducing psychoactive chemicals.

A total of 120 children and teenagers aged two to 18 were recruited for the study in the US and Europe.

Patients were given twice daily doses of liquid CBD, taken by mouth, or an inactive placebo treatment for 14 weeks.

For those given the cannabis drug, the typical number of convulsive seizures suffered per month was halved from around 12 to six.

The percentage of patients who experienced at least a 50 percent reduction in the frequency of convulsive seizures was 43 percent for the CBD group and 27 percent for the placebo group.

Five percent of young people on the cannabinoid treatment were seizure-free by the end of the trial.

But it is thought that while the drug alleviates symptoms as long as it is being taken, it does not offer a cure.

The study's other lead author and consultant in paediatric neurology at Great Ormond Street children's hospital in London, Professor Helen Cross, said: "The results ... are significant and provide us with firm evidence of the effectiveness of cannabidiol."

She thought the drug had made a "major difference" to the lives of the families of patients taking part in the trial.

"For those families for whom convulsive seizures have been reduced, they are able to go out more; the children are more awake, in their opinion, and more interactive," said Prof. Cross at a news briefing in London.

Results from the trial are reported in the New England Journal of Medicine.

Side effects from the treatment were said to be mild or moderate in severity, most commonly involving vomiting, fatigue and fever. Eight patients from the CBD group and one taking the placebo dropped out of the trial due to side effects.

Experts still do not understand precisely how the cannabis compound controls epilepsy symptoms. It is thought to act on the endocannabinoid system, a network of molecular receptors in the brain and nervous system that not only generates the cannabis "high" but is linked to a wide range of physiological processes.

Chief executive of Great Ormond Street Hospital Dr Peter Steer said: "The importance of this research cannot be underestimated. It is the first time that a drug has been identified which can significantly reduce the risk of seizures for children living with the debilitating Dravet syndrome of epilepsy."

The drug's manufacturer, Cambridge-based GW Pharmaceuticals, funded the trial which was carried out by independent researchers.

The compound, given the brand name Epidiolex, is the second cannabis-derived medicine produced by the company.

In 2010 GW Pharmaceuticals launched the world's first prescription cannabis drug, the multiple sclerosis treatment Sativex.