Magical addiction manifests itself as an utter consumption with the concept of casting spells. It can attack any spellcasters who spend more than one week on the Astral Plane, as long as they have used any amount of magic at all while there. If this is the case, they will have a 1% chance per day of succumbing to addiction. This chance drops to once every other day if their Wisdom is over 13, once per week if over 16, and no chance at all if it is over 18.
After leaving the Astral, the caster can find joy only while in the act of casting, and they constantly possess a yearning to return to the Astral, where magic seems more pure. In any sort of threatening situation, their first response is to cast a spell, and even during calm times, they must keep themselves in check to refrain from using magic. (Will save vs. DC 10 once per hour to keep from using a spell.) If they exhaust their supply of magic, they become utterly despondent (-1 to all actions) until they have the opportunity to rest and replenish themselves. Magical items that grant the ability to cast a spell can be used at any time in lieu of an actual spell.
This ailment can only be cured through magical means, through the application of a restoration spell, psychic surgery, or some similar sanity-restoring effect.