I think most of us, if presented with the situation at the beginning of a relationship, would run a mile from being involved with an addict. However, addiction can present itself even in a long term relationship and your decisions are a bit more complicated at that point.
In case you don’t know the downside of being involved with an addict, it involves feeling as though you’re slowly losing your mind because your possessions keep disappearing, feeling alienated from your partner as their priorities shift from your relationship to their addiction, and feeling despair as you feel increasing isolated and stressed out as you attempt to keep your life together while not letting anyone know what’s going on. Suffice it to say, it isn’t pretty.
If you find that your significant other is an addict, you must insist that they get help. While it is true that an addict will not maintain long-term sobriety based on someone else’s desires, you can not remain in a relationship where the addiction, not you, is the top priority. If they refuse to get help (and real help, not just a promise to stay away from it) you need to consider your options, both financially (you may wish to separate assets) and in the relationship. You will also need to get help for yourself – a counselor to talk to, a legal adviser, and a strong support system to help you through the worst of it.