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.
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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.