A new study suggests cannabidiol, a derivative of marijuana, may help some children with severe epilepsy to make recovery, npr reports.

The  study is being presented at the American Epilepsy Society meeting in Philadelphia this week.

The study commenced in 2014 with 313 children from 16 different epilepsy centers around the country.

During the three-month trial, 16 percent of the participants left because the cannabidiol was either ineffective or had adverse side-effects, said Dr. Orrin Devinsky, a neurologist at the New York University Langone Medical Center and lead author on the study.

However, the 261 patients who continued taking cannabidiol experienced a decrease in their convulsive seizures by about half on an average.

Devinsky said that some children continued to benefit even after the trial ended.

"In the subsequent periods, which are very encouraging, 9 percent of all patients and 13 percent of those with Dravet Syndrome epilepsy were seizure-free. Many have never been seizure-free before," he says.

However, Devinsky and others say it's impossible to tell if cannabidiol is having a real effect on epilepsy without full clinical trials, since the effects of the drug might be a placebo effect or could be attributed to some other factor that hasn't been caught in the study.

Also, doctors will need to see what happens when a patient is on cannabidiol for more than a few months.