Real evidence of marijuana’s dangers should not get lost in the weeds.

A second independent study released in less than a month offers serious warnings that even so-called medical marijuana easily can do more harm than good. The first lesson from these studies ought to be that lawmakers should not make marijuana generally legal and that they should go slow, with stringent regulations, even in legalizing medical marijuana. The second lesson is that even when legal, doctors should be wary of prescribing it.