Mana Sickness is a debuff that reduces the player's magic damage after they consume any mana-restoring item. Automatically drinking Mana Potions with the Mana Flower will also cause the debuff. Whenever a mana-restoring item is consumed, the player is inflicted with 5 seconds of Mana Sickness[1]. The percentage of magic damage reduction is proportional to the remaining debuff duration (see Notes for formula), reducing by 1% every 1/5 second; at 5 seconds remaining, the damage reduction is 25%. All mana-restoring items inflict the same length of the debuff; for example, a Lesser Mana Potion induces the same duration of Mana Sickness as a Greater Mana Potion.
(Desktop, Console and Mobile versions) If the player consumes additional mana-restoring items while the debuff is active, the duration will be increased by 5 seconds for each item consumed, up to a maximum of 10 seconds (50% damage reduction).[2]
The magic damage reduction is multiplicative rather than additive, and is applied after all damage multipliers. For example, if the player has +100% damage from buffs and items, the total damage with the debuff at 5 seconds left comes to 150% (200% × (1 - 25%); -25% comes from Mana Sickness).
Desktop 1.3.0.1: Now the duration stacks and damage-reducing effect increases if mana-restoring items are consumed while the debuff is active, up to 10 seconds and 50% lowered magic damage.
↑Information taken from the Desktop1.4.4.9 source code, method ApplyLifeAndOrMana() in Terraria.Player.cs and field manaSickTime in Terraria.Player.cs.
↑Information taken from the Desktop1.4.4.9 source code, method AddBuff() in Terraria.Player.cs and field manaSickTimeMax in Terraria.Player.cs.
↑A tick is a time unit countable by the software. Most of Terraria's updating logic happens every tick. A tick has the length of 1/60th of a second, hence there are 60 ticks in a second and 3600 ticks in a minute.
↑Information taken from the Desktop1.4.4.9 source code, method UpdateBuffs() in Terraria.Player.cs and field manaSickLessDmg in Terraria.Player.cs.