The drug, GW Pharmaceuticals' Epidiolex, is made of cannabidiol, or CBD, a component of marijuana that does not give users a high. It is given as an oil, and in clinical trials, it was shown to reduce1 the number of seizures by about 40 percent in patients with Dravet or Lennox-Gastaut syndromes.

"This approval serves as a reminder that advancing sound development programs that properly evaluate active ingredients contained in marijuana can lead to important medical therapies," FDA Commissioner Scott Gottlieb said in a statement. "And, the FDA is committed to this kind of careful scientific research and drug development."

The FDA's decision was expected. FDA officials had indicated they supported approving Epidiolex, and an advisory panel had unanimously recommended it get the green light.