generosity/giving help to those in need is a good trait. don't lose that in your pursuit of not being taken advantage of.
but in my own experience aiding alcoholics, they are not to be trusted to reciprocate. it's not necessarily that alcoholics are bad people, but that their addictions are stronger than they are. at this level, it is important to not expect anything in return, or even, expect to be used. it's a sad truth of life.
just be careful, i guess, is the best advice.
and keep being a nice person as well as you can.