This is mentioned in the modified spells topic
Quote:
Heal
If cast via an item (e.g. a potion), successive uses of the spell reduces the amount it heals, depending on the last time it was cast. Additionally, it will only heal a maximum of 80% of a character's health points if cast this way. If cast the round before, it will only heal 5% maximum hit points. If cast two rounds before, it will only heal 10% maximum hit points. This continues up to fifteen rounds, where it will heal 80% maximum hit points once more.