Dravet syndrome is a rare genetic epileptic encephalopathy (dysfunction of the brain) that begins in the first year of life in an otherwise healthy infant.
Cannabidiol, or CBD, is a compound in the cannabis plant that does not contain psychoactive properties that induce a high.
"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," said Orrin Devinsky, professor at New York University Langone Medical Centre in the US.
Participants were randomised across 23 sites in the US and Europe to receive either CBD 20 milligrammes/kilogrammes or placebo was added to their existing treatment over a 14- week period.
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Seizure frequency was tracked for one month prior to and during the course of the study for baseline readings.
Researchers found that seizure frequency dropped in the CBS-treated group by 39 per cent from a median of nearly 12 convulsive seizures per month before the study to about 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, researchers said.
Side effects - experienced by 93.4 per cent of patients in the BCD group and 74.6 per cent of those treated with placebo - were generally reported as mild or moderate in severity.
The most common side effects in the BCD group were vomiting, fatigue and fever, researchers said.
The study was published in the New England Journal of Medicine.