Our most popular and addictive drugs come from plant toxins; caffeine, tobacco, cannabis, cocaine, heroin, are all derived from what are supposed to be poisons. These toxins were developed by plants to ward off herbivores who would otherwise eat them. So why is it that we not only tolerate them, but have found ourselves in a position of craving them, sometimes desperately? It is a paradox at which researchers are taking a fresh look.