The Calming Potion is a buff potion which grants the Calmbuff when consumed. While under its effects, the enemy spawn rate is reduced by 17% and the max amount of enemies on screen is reduced by 20%. This lasts for 8 minutes, but can be canceled at any time by right-clicking the icon (), by selecting the icon and canceling it in the equipment menu (), by double-tapping the buff icon (), or by canceling the buff from the buffs screen ().
If the player wants to loot the chests in the Dungeon and already has a sufficient number of Golden Keys, using this potion before entering may be a good idea.
While not directly helpful against bosses themselves, it can be useful to use against some like the Queen Bee or the Wall of Flesh whose biomes typically have numerous enemy spawns which could make the fight harder, or when mining Meteorite to curb Meteor Head spawns.