I seriously doubt San Diego has the worst drug problem, but the biggest drug addict ever was a suburbanite white girl in East County, the hick and hillbilly area. She overdosed on heroin a couple months ago. Serves her right. She was also a horrible fukking human being, working out of a heroin lab out in Tecate.
But if I had to take a stab at the druggiest city? Probably Camden, NJ. I know it's small, but if you're looking at concentration of drugs and not just overall number, I would say that Camden is a safe bet.
I don't look at all drugs in the same light, though. I mean, shyt, there's weed which is
technically a drug, but nobody under the age of 70 really looks at it that way. Then there's stuff like e and MDMA, both of which are considered happier, party drugs. Then there's coke, which is kind of a rich a$$hole/party drug. Then there's the hippie drugs like acid, shrooms, and DMT.
Then there's shyt I will never touch in a million years like heroin, meth, etc.
Here's an interesting and somewhat related infographic (download the image if it doesn't fit in your browser):