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.  NOVA investigates the story of cannabis in the United States, from the criminalization that has disproportionately harmed communities of color to the latest medical understanding of the plant.