A massive social experiment is underway.

Around 55 million Americans use cannabis today, with numbers rising as legalization expands. Yet it remains federally classified as a Schedule I drug—defined as having no accepted medical use—which has limited scientific research into its complex effects.

Containing over 100 cannabinoids, cannabis includes THC, responsible for its high, and CBD, being studied for potential therapeutic benefits such as epilepsy, PTSD, and insomnia.

NOVA explores the story of cannabis in the U.S.—from criminalization and its uneven social impact to the emerging science of its risks and potential medical uses.

What are the effects on the developing brain, and what does long-term use really mean? As acceptance grows, science is racing to catch up.