The decision is the latest sign that federal health officials want more research on medical marijuana as its use grows.

More than 300 veterans diagnosed with post-traumatic stress disorder will take part in a five-week study approved by the Food and Drug Administration to examine the effects of smoking cannabis on symptoms from the illness.

The move marks a rare action by the FDA to sanction a study on cannabis despite the plant’s current Schedule I classification under federal law as a drug with no therapeutic use, in the same category as heroin and LSD.

The Food and Drug Administration (FDA) this week authorized a long-delayed clinical trial to test whether smoking marijuana can be used to treat post-traumatic stress disorder (PTSD) in veterans. The trial,