Addicting and Corrupting Magic

[Gildedragon]

Depends what system you're using. I am most familiar with 3.P so...

Taint mechanics in Heroes of Horror might be a good baseline. So we'll work with those for now.

Next: what sort of campaign you want to run; thinking a sorta horroresque one, where the bonuses per dose start strong but weaken as the addiction progresses.

Putting it together:

The Substance IS magic, pure and powerful, or so the dealers say. It is unquestionable that it increases the might of the user; but at what cost?

Each dose of substance grants a bonus (see below) but carries a risk with it; immediately after consumption one must make a Fort save with the DC increasing a lot with each prior usage.
Failure means they gain 1 point of corruption, which affects Con or Wis negatively, making more corruption gain easy.
The bonuses are temporary and stacking
At first stages: +1 or +2 self-stacking profane bonus to Cha, +4 spell levels to cast (can be used for metamagic-ing spells beyond actual casting power), a +8 CL bonus to next spell; the bonuses last for 24 hours or until the bonus spell levels are used up
Then the bonuses are halved, then quarterd, or limited to which one can get (instead of getting all 3 you only get 1 per dose) and so on.
Withdrawal involves a daily check with the save the Substance has gutted, failure to imbibe results in Cha damage which is healed once the Substance is once again consumed. N failed checks (as many as your corruption) or one successful one result in lowering your corruption by 1.
... Heading out but this is a rough draft of how it might work in 3.5