noun
magic potion
A potion with magical effects, commonly found in fantasy settings.
魔法薬を飲むと、傷がたちまち治った。
When I drank the magic potion, my wounds healed instantly.
薬 is a general term for medicine or drugs, whereas 魔法薬 is a fantasy item.
Compound of 魔法 (magic) and 薬 (medicine/drug). The exact historical origin is uncertain, but it is a straightforward modern coinage for fantasy contexts.