WASHINGTON — The U.S. Food and Drug Administration has approved a marijuana-derived drug for the treatment of two rare and serious forms of epilepsy, Lennox-Gastaut syndrome and Dravet syndrome, that begin in childhood but can persist in adulthood.

The drug is made from purified cannabidiol (CBD), a compound found in the cannabis plant. The drug will be marketed under the brand name Epidiolex.

CBD has medicinal effects, but it does not cause the mind-altering high that comes from THC, the primary psychoactive component of marijuana.

The FDA said this is the first drug approved in the U.S. that contains a purified substance derived from marijuana. The agency has previously approved drugs made from synthetic versions of THC and other marijuana constituents.

British drugmaker GW Pharmaceuticals studied the drug in more than 500 children and adults with hard-to-treat seizures, overcoming numerous legal hurdles that have long stymied research into cannabis.

“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,” said FDA Commissioner Scott Gottlieb.

The FDA has previously approved synthetic versions of another cannabis ingredient for medical use, including severe weight loss in patients with HIV.