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As of patch 2.1, all potions and elixirs that added a buff were designated as either Guardian Elixirs or Battle Elixirs. The Guardian Elixirs being those that increase your defense, stamina, protection, mana regen, armor, etc. Elixirs will stack with ability-generated buffs (Arcane Intellect for example) but will not stack with other Guardian Elixirs.

Players can only have one Guardian Elixir and one Battle Elixir active on them at one time, or one Flask, which counts as both Elixir slots.

Review the Elixir article for a full list of items classified in-game as Guardian Elixirs.