GW Pharmaceuticals' cannabis-derived medicine for severe childhood epilepsy won a favorable review from U.S. Food and Drug Administration (FDA) staff on Tuesday, boosting hopes for its approval in the world's biggest drugs market.

Shares in the company, which has operations in Britain and the United States, were 11 percent higher in early Nasdaq trade on the news.

GW's medicine Epidiolex, which is given as a syrup, is a purified form of cannabidiol, one of the active ingredients found in marijuana. It contains less than 0.1 percent of tetrahydrocannabinol, the substance that makes people high.

It is designed to treat Dravet syndrome (DS) and Lennox-Gastaut syndrome (LGS), rare childhood-onset forms of epilepsy that are among the most resistant to treatment.

FDA staff said overall findings from three pivotal clinical studies demonstrated that, when added to a current therapy, GW's drug "reduces seizure frequency in patients with drug-resistant LGS or DS while maintaining a predictable and manageable safety profile".

The FDA briefing document, posted on the agency's website was prepared ahead of an advisory committee meeting on April 19.

An editorial in the New England Journal of Medicine last year said that clinical evidence of Epidiolex's efficacy represented "the beginning of solid evidence for the use of cannabinoids in epilepsy" after an era of anecdote.