How about some love for Pathfinder for spell sickened heroes with a drawback?
Spells hurt you almost as much as they do your enemies. You have learned to cast them with care.
Effect: When you successfully cast a spell with out an item, you must attempt a Fortitude saving throw. The DC is 10 + level of the spell. If you fail this save, you become sickened for 10 minutes. If you case another spell while sickened from this effect and fail the fortitude save, you instead become nauseated for 10 minutes. At the end of each round of combat while nauseated form this effect, you may attempt another fortitude saving throw for the second spell. If you pass, you become sickened instead.