Sometimes, but mainly it's because people have been through some stuff that made them turn to drugs.
I think addiction is just something that people choose to do when they feel as if there is no other way to handle a certain situation. Addiction is basically a weakness, some people aren't that strong to get through them so usually it has nothing to do with your intelligence

Good question. I do not believe there is a simple answer. However, I believe the more intelligent you are the more internal and external pressure you face which drives you to vices to cope with the burden of your brilliance.  Also, the more intelligent you are the more you realize how flawed the paradigms and beliefs structures that make up our world are, which is another reason to seek relief in mind/mood altering chemicals. Some people just do not understand what the burden of massive potential feels like.

Imo from seeing it all up close addictions start from anything to pain meds from serious (sometimes) injuries, to a depressed state that the user feels needs to be medicated or self medicated, or even just to feel high out of misguided(?) boredom. This latter example may be where the idea that intelligence and addiction are directly linked comes from. However thats just what I've gathered and I am in no way a certified expert. So yea. Just rambling.