4. Marijuana

Cannabis is the most widely used illicit drug on the planet. To many, it's an evil weed, but to some, it's a sacred herb, even a lifesaver. For decades the global supply was controlled by criminals, but now a quasi-legal industry worth billions of dollars is booming. Visit growers who are leading a horticultural revolution, hear from users consuming for medicinal purposes, go inside the "Green Rush" of medical marijuana in cities across the U.S. and the violent marijuana cartels in Mexico.