Okay, I suggest you look at the different drugs available in the BoVD and use a similar mechanic the cancer mage did with diseases. They are immune to any ability damage that comes first in either the initial and secondary effects of their chosen drug, however not the overdose. They also do not suffer the ability damage on the addiction however will automatically inact 24 hours if they hadn't taken the drug within that time. They obviously get different bonuses and may allow some stacking of benefits.
