On a societal level of health problems, smoking definitely far exceeds anything else. Alcohol is definitely second place, and if you get truly addicted to it (alcoholism), it damages one's social life and work life in a way that looks a heck of a lot like addictions to some illegal substances. It also can cause health problems, especially for alcoholics. Meth is the worst addiction as far as illegal drugs go here in the states. However, as for all addictions that people can have, Krokodil is definitely the worst by a long shot. I'm not even going to talk about it on here as it could be very disturbing for some people to even hear about. However, if you are curious what it is, google it. Just be warned, it is VERY GRISLY AND NOT FOR THE SQUEAMISH! When I heard about it, I was truly in disbelief that any human being would ever consume this substance! At first I thought it might be a hoax, but shockingly it isn't!