One of the central issues in the debate over the medical-marijuana question on Tuesday’s election ballot is whether scientific research confirms that marijuana can offer health benefits.

Up to now, a marijuana-derived drug had not been approved as a medicine by the Food and Drug Administration. But on Monday, the agency approved the first such drug, called Epidiolex, which contains cannabidiol and will be used to treat two rare forms of epilepsy in children. The drug does not have the properties that make people high.

The FDA requires clinical trials in hundreds, then thousands, of human patie...