The Potion is an item given to a player by asking the Apothecary in Varrock for a free sample, with the Apothecary choosing to give it to players roughly half the time and refusing other times. Before Varrock was updated, the potion was available by way of a respawn in the apothecary's building. It has only a few functions in RuneScape; it is a possible item requirement for the Forgettable Tale of a Drunken Dwarf quest.
If one uses it on a slave in Sophanem, they get a little bonus chat. If one uses it on the [head] Priest in Sophanem they get a second bonus chat. Using it on the other human NPCs gets the "Nothing interesting happens." message. If you use it on one of the plague cows, they will say "Moo". Using it on the plague frogs will not result in anything happening.