The only problem I have with this is if it's any sort of illegal drug. If he can only draw power from, say, weed, that would somewhat make sense. However, if he can draw power from ANY illegal drug, then that's a no. Why would the drug being illegal change the way his body reacts to it, you know? 

I would also suggest something like pain pills that ARE legal, but only if you have the proper prescription. Still illegal and pretty sketch, but not so bad that the Guild can't turn a blind eye for the greater good.