A huge social experiment is underway. Fifty-five million Americans say they currently use cannabis, and that number is expected to grow as voters push for legalization in more and more states. Though many believe cannabis is benign, and even beneficial, the federal government classifies it—like heroin—as a Schedule 1 drug, one that has no medical uses and poses serious safety concerns. Because of its classification, research on the plant’s complex chemical makeup—and its effects on users—is lagging. Cannabis contains 100 different cannabinoids, chemicals that mimic ones found throughout our bodies. One is THC, which produces the high associated with cannabis use. Another, called CBD, shows promise for treating seizures, PTSD, opioid addiction, and insomnia. NOVA investigates the story of cannabis in the U.S., from the criminalization that has disproportionately harmed communities of color to the latest medical understanding of the plant. What risks does cannabis pose to the developing brain? How much do we know about its potential medical benefits? As cannabis becomes socially accepted, scientists are racing to understand its long-term health consequences.