May 14, 2015 11:05 AM EDT

This week’s TIME cover story examines how little we actually know about marijuana’s effects on our brains, even as cities and states around the United States are legalizing the drug.

It’s not the first time marijuana has been cover-story material. TIME has published five cover stories about the drug that, taken together, show how society’s view of marijuana has evolved over two decades.

Read the new cover story here, on The Great Pot Experiment

Write to Lily Rothman at