U.S. health regulators have approved the first prescription drug made from marijuana, a milestone that could spur more research into a drug that remains illegal under federal law.

The Food and Drug Administration approved the medication from GW Pharmaceuticals on Monday to treat epilepsy.

The drug's approval permits its use in patients aged two years and older with Dravet Syndrome (DS) and Lennox-Gastaut Syndrome (LGS), rare childhood-onset forms of epilepsy that are among the most resistant to treatment.

The strawberry-flavored syrup is a purified form of a chemical in the cannabis plant — but not the one that gets users high. It's not yet clear why the ingredient, known as CBD, reduces seizures in some people.

The British drugmaker studied the drug in more than 500 patients with hard-to-treat seizures, overcoming numerous legal hurdles to conducting research with cannabis.

FDA officials said the drug reduced seizures when combined with older epilepsy drugs.

The drug, called Epidiolex, is derived from cannabidiol (CBD), one of the hundreds of molecules found in the marijuana plant, and contains less than 0.1 percent of tetrahydrocannabinol (THC), the psychoactive component that makes people high.

Although THC can induce paranoia, anxiety and hallucinations, CBD has the opposite effects and has been cited by scientists as a potential treatment for mental health issues.

"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.

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