Reclassifying marijuana as just another drug—instead of a dangerous one—won’t legalize weed everywhere in the US, but it acknowledges market reality and may set the stage for both deeper research of its effects and a commercial boom.

Since then, dozens of states have legalized the drug for a variety of uses, creating an unwieldy patchwork of laws and regulations. A multibillion-dollar legal cannabis market has arisen, with specia…