Lesser Mana Potion

Drinking one cuts your magic damage for a stretch afterward, and drinking a second while the first penalty is running makes it worse. This is for the moment you run dry in the middle of a fight, not something to sip between casts. Between fights, stand still and let regeneration do the work for free.

The bottles sitting on Dungeon platforms are the real supply for most players. They read as a Placed Bottle on the map and function as one, so you can brew next to them, and a pickaxe or a right click frees the potion to carry away. Between those and the crates you pull out of the water, you rarely need to buy any.

The larger tiers restore more for the same sickness penalty, and that ratio is the only thing that matters once you are in Hardmode. Keep the small ones for early caving and stop carrying them the moment better ones exist, because the debuff is identical either way.