There are a great number of problems with dating a person involved in chemical dependency. I want to note that I am not using the word uses drugs. I am using the words chemical dependency.
First we have to classify what is going on. Are we talking about steroids, are we talking about meth, are we talking about pot? Are we talking about a guy who buys generic viagra off the internet. First it is a matter of definition.
If we are talking things like coke, meth, opiates, Rx pain killers, etc. those can be horrific dependencies and not only place the life of the person using these substances recreationally in jeopardy, the jeopardize your life as well.
The same can be said for a practicing alcoholic.
In the years I lived in Palm Springs as it became a "disease factory" I watched numerous men I knew become intoxicated via different substances. Those who had addiction problems to chemical substances almost universally were sero-converted from negative to positive status before the end of the first year. Nobody was deliberately bad, what happened was that people's brains were influenced or clouded by the chemicals and they willingly exposed themselves to infection in such a manner as to become infected.
Personally, a practicing chemical abuser is a "wild-card" and neither you nor that person truly know what is going on at all times in their life. It is common when a "chemhead" wants sex or anything else that they depend on for them to obtain that using sex as a reward to the individual or individuals who obtained or secured what they wanted.
I watched this more than once. It would be just to dangerous in my eyes.