An epilepsy treatment that uses a form of marijuana just took a critical step toward approval in the US

The experimental cannabis-based drug has successfully treated children with a rare form of severe epilepsy called Dravet syndrome in a late-stage clinical trial. The news sent shares of GW Pharmaceuticals, the company developing the drug, up more than 100% on Monday.

It's the first of four final-stage Phase 3 epilepsy trials that the drugmaker hopes will confirm the therapeutic benefits of cannabinoids, the active ingredients found in marijuana.

GW said the 120-patient trial showed patients taking the drug, Epidiolex, achieved a median reduction in monthly convulsive seizures of 39% compared with a reduction on placebo of 13 percent, which was highly statistically significant.

In light of this positive data, Chief Executive Justin Gover said the company would now request a meeting with the U.S. Food and Drug Administration (FDA) to discuss its plans to seek regulatory approval for treating this particular form of epilepsy.

There are currently no FDA-approved therapies for Dravet syndrome, a lifelong epileptic condition that begins in infancy.

GW has already won approval for its multiple sclerosis treatment Sativex, in more than 20 countries. The US has not approved the drug.

Epidiolex, however, is commercially more significant since GW retains full control of the product and the company has also geared the medicine's development to the big U.S. marketplace.

There's optimism about the new drug's prospects has been building following positive feedback from "compassionate access" programs involving hundreds of American children, but the rest of the Phase 3 program will either make or break the success of the drug.

(Reuters reporting by Ben Hirschler)