How would you handle the aftereffects of magic?

[Elder-Basilisk]

And then you can have the spells start lasting for shorter and shorter periods of time. After a few uses, spells that used to last one hour/level, will only last 1 minute/level so the characters have to work harder and harder to make that high last--and it won't even be as high as the older empowered versions.

Hey, this really is starting to sound like drug addictions.

Or maybe becoming addicted to a Cat's Grace or Bull's Strength effect; you'll do anything, just so long as you don't stay as slow as you were. Works especially well when you're also using mental boosters, because then you start to get paranoid; you need those spells, because you never know when they're gonna come after you.