Is there truly such a thing as addiction to a drug or chemical? I understand that taking drugs like heroin can change the physiology such that it's unpleasant to quit. But if a narcotic addict (or alcoholic) really wants to quit, it seems that the real obstacle is changing one's lifestyle and breaking a bunch of bad habits -- not just getting chemically "clean." Similarly, can a person be addicted to certain foods such as sugar or wheat, or it is merely just a lack of willpower? Finally, some people claim to be "addicted" to gambling or the internet. Is that true? Is there any merit to the notion of an "addiction-prone personality"?