"We still need more research, but this new trial provides more evidence than we have ever had of cannabidiol's effectiveness as a medication for treatment-resistant epilepsy."

The study included a liquid pharmaceutical formulation of CBD, called Epidiolex, which has not been approved by the US Food and Drug Administration.

For the study, 120 youngsters aged from two to 18 with Dravet Syndrome were randomised across 23 sites in the US and Europe to receive either CBD 20 mg/kg or a placebo added to their existing treatment over a 14-week period.

Seizure frequency was tracked for a month prior to the study, and during the course of the study.

Specifically, seizure frequency dropped in the CBD-treated group by 39 per cent from a median of nearly 12 convulsive seizures per month before the study to around six; three patients' seizures stopped entirely.

In the placebo group, there was a 13 per cent reduction in seizures from about 15 monthly seizures to 14.

The difference in the degree of seizure reduction between the CBD group and the placebo group was both statistically significant and clinically consistent, according to the researchers.

Side effects - experienced by 93.4 per cent of patients in the CBD group and 74.6 per cent of those treated with a placebo - were generally reported as mild or moderate in severity.