Researchers in the United States say they have found a medicine made from marijuana that won't give users a 'high' but will cut seizures in children with a severe form of epilepsy.

"This is the first solid, rigorously obtained scientific data that a marijuana compound is safe and effective for this problem," said one study leader Dr Orrin Devinsky of NYU Langone Medical Center.

The study was published on Wednesday in the New England Journal of Medicine.

Dr Devinsky said research into promising medical uses has been hampered by scientists having to get special licenses, plus legal constraints and false notions of how risky marijuana is.

"Opiates kill over 30,000 Americans a year, alcohol kills over 80,000 a year. And marijuana, as best we know, probably kills less than 50 people a year," Dr Devinsky said.

For years, desperate patients and parents have argued for more research and wider access to marijuana, with only anecdotal stories and small, flawed studies on their side. The new study is the first large, rigorous test - one group got the drug, another got a dummy version, and neither patients, parents nor doctors knew who took what until the study ended.

It tested a liquid form of cannabidiol, one of marijuana's more than 100 ingredients, called Epidiolex. It doesn't contain THC, the hallucinogenic ingredient, and is not sold anywhere yet, although its maker, GW Pharmaceuticals of London, is seeking US Food and Drug Administration approval.

The company paid for, designed and helped run the study, and another doctor involved in the study has related patents.

Patients in the study have Dravet syndrome , a type of epilepsy usually caused by a faulty gene. It starts in infancy and causes frequent seizures, some so long-lasting they require emergency care and can be fatal. Children develop poorly, and their mental impairment seems related to the frequency of seizures - from 4 to as many as 1,717 a month in this study.

The study included 120 children and teens, aged from two to 18, in the US and Europe. They took about a teaspoon of a sweet-smelling oil twice a day (drug or placebo) plus their usual anti-seizure medicines for 14 weeks. Their symptoms were compared to the previous four weeks.

Serious seizures with convulsions dropped from around 12 a month to about six for those on the drug and did not change in the others. Three patients on the drug became seizure-free during the study.

It's no panacea, though. Diarrhea, vomiting, fatigue, sleep problems and other issues were more frequent in the drug group. Twelve patients quit the study - nine on the drug and three in the placebo group.