A number of states, in response to pressure from these parents, have passed or considered legislation to make it easier to obtain marijuana-based products. And some families have become “marijuana refugees,” moving to Colorado where it has been easier to obtain a particular extract, known as Charlotte’s Web, after the girl who first used it to control seizures.

Hundreds of other children and young adults have been using Epidiolex outside of clinical trials, under programs that allow desperate patients to use experimental drugs.

While many parents have reported significant reductions in seizures, experts have been cautious about anecdotal reports, saying that such treatments needed to be compared with a placebo to make sure they work. As such, the results from the GW trial have been closely watched.

“I’m very proud and happy about this study because it is science — we did things the way they should be done,” the study’s lead investigator, Dr. Orrin Devinsky of the Comprehensive Epilepsy Center at New York University Langone Medical Center, said in an interview. “I would strongly advocate that in the United States we need to do systematic assessments of medical marijuana.”

The study involved 120 patients with an average age of 10 and an average frequency of 13 convulsive seizures a month at the start of the study, despite taking an average of three other drugs. Half of the children were randomly assigned to take the drug and the other half the placebo, in addition to the epilepsy medicines they were already taking.