Cross addiction, particularly substitute drug use, is always a problem. The DSM-IV lays out a number of criteria that outline what addiction is. Perhaps the most central element that defines addiction is the ongoing use of something, despite harmful consequences. I might eat tuna fish everyday, but if it's not making me sick or hurting my relationships with others, then daily use of tuna fish is no big deal. But if my daily marijuana use is causing me to be intoxicated while driving, and becoming more withdrawn from important relationships, then continuing to use marijuana despite those consequences, I am demonstrating addiction.
Some people become addicted quite rapidly.
There's more and more research that suggest that for some individuals, there is a genetic predisposition for addiction. In other words, their brain chemistry makes them that much more susceptible to becoming addicted to alcohol or other mood altering substances. In the case of these people, we'll often see rapid onset of addiction symptomatology shortly after they begin to use mood altering substances. Others, however, become addicted after many yeas of continued use of a mood altering substance. These folks are not born with a predisposition, but through regular and continued use, neurochemistry has been altered and they have subsequently begun to demonstrate the patterns of addiction.