The Food and Drug Administration on Monday approved the country’s first drug derived from marijuana, a medication that treats two rare and devastating forms of epilepsy.

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 reduce the number of seizures by about 40 percent in patients with Dravet or Lennox-Gastaut syndromes.