Rubella is an ancient medicine that was used to increase the physical strength of a person in times of war.
It can enhance the strength of the person to the maximum, even if the body of the person is breaking apart. The main purpose of this medicine is to make the person forget the original pain. It is like making a person immune to all the pain. That is what made the drug illegal and used during the war to increase the strength of the empire's army. The reason it became illegal is that after the war when the Kingdom of Anthia lost, the emperor of the winning nation termed it illegal. Since then, it has become rare for an herb to be found in a nation.
Anyone will be curious why dangerous stuff like rubella can be found in a back alley that easily and how Minella knows about that.