functioning addict
A person that has an addiction whether it be to alcohol or drugs and still able to maintain a "normal" life.
My boyfriend is a functioning addict. He can drink from the time he wakes up until the time he goes to bed and still is able to go to work.
functional addict
An individual to spite of having a chemical dependency, is able to maintain a career, family setting, and functioning semi normally into society. Many of the times these individuals require their drugs in order to function in society.
Yes I'm strung out I fix many times a day but I live in a nice house I have a loving wife and my children never want for anything I am the most functional addict there is.