Horizon: Cannabis: The Evil Weed?

Cannabis is the world's favourite drug, but also one of the least well understood. Can cannabis cause schizophrenia? Is it addictive? Can it lead you on to harder drugs? Or is it simply a herb, an undervalued medicine? Addiction specialist Dr John Marsden discovers that modern science is finally beginning to find answers to these questions. John traces the cannabis plants birthplace in Kazakhstan; finds the origins of our sensitivity to cannabis in the simple sea-squirt; and finds out just what it does to our brains.