These days, it’s hard to miss cannabis dispensaries. In the 40 states (plus D.C.) that have legalized medical marijuana, storefronts and advertisements promote cannabis as a treatment for virtually every condition, from chronic pain to anxiety to (ironically) drug addiction. Many consumers appear persuaded. In one survey, more than a quarter of adults in the United States and Canada have tried cannabis for medical purposes.

But how well-supported are these claims? A new review published in JAMA, which attempted to examine every high-quality study from the last 15 years alongside guidelines from major medical societies, came to a sobering conclusion: For most uses, the evidence was limited or inconclusive. In many cases, medical organizations recommend against using cannabis because the evidence for harm outweighed potential benefits.

The review was careful to distinguish cannabis from specific cannabinoids. Cannabis refers to the plant and its various forms, including dried leaves and extracts, which contain dozens of biologically active compounds in unpredictable amounts. Cannabinoids are the individual chemical components, some of which have been rigorously studied and approved by the Food and Drug Administration for specific medical uses.

These approved uses include treatments for severe nausea and vomiting caused by chemotherapy,........

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