Rat poison is a quest item in the Clock Tower and Ratcatchers quests.
In the Clock Tower quest, players can pick up the rat poison spawn in the basement of the Clock Tower before retrieving the white cog. It is used on the rats' feeding trough to poison them, killing them, thus opening the western door to pick up the white cog.
In the Ratcatchers quest, if the player has a vial, kwuarm, and red spiders' eggs, after talking about the rat infestation, Hooknosed Jack will make rat poison to use on four pieces of cheese to make poisoned cheese. He will not make rat poison for the player after this quest.