For the first time, scientists have demonstrated that a component of cannabis reduces seizures in children with a rare form of epilepsy, marking a significant step in efforts to use marijuana and its derivatives to treat serious medical conditions.

The company that sponsored the Phase 3 trial, GW Pharmaceuticals, had already announced some of the results, but researchers said the full peer-reviewed study, published Wednesday in the New England Journal of Medicine, validated the importance of the research. They also pointed out that the drug, cannabidiol, helped some patients more than others and was associated with a range of sometimes severe side effects, a significant finding because some families have been treating their children on their own in states where recreational marijuana use is legal.