Greater Mana Potion

You cannot craft your way into this tier, so the Wizard is your supply line and you buy in bulk before a boss rather than scrambling mid fight. Biome Mimics drop them as well, which is usually where a first stack arrives if you have not gone looking for the Wizard yet. Finding him is the real errand.

Every drink cuts your magic damage for a window afterward, so gulping one the instant your bar bottoms out is often worse than waiting a beat for natural regeneration to catch up. The right moment is a lull, or any stretch where you would be doing nothing anyway. A Mana Flower automates the drinking, which is convenient and also means the penalty fires whether you wanted it to or not. Wearing one through a damage window you cannot afford to weaken is a mistake.

Star statues, mana regeneration accessories and Nebula armor all restore mana without touching the sickness at all. If your build supports one of them, potions drop back to being an emergency button instead of part of the rotation.