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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 two rare forms of childhood epilepsy.

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.