In "The Cask of Amontillado" Montresor gives Fortunato a draught of Medoc. How does it help, and how does it harm him?
The author Edgar Allan Poe sets up certain conditions which help Montresor to achieve his purpose of leading Fortunato to his death. Perhaps the most important one is that Fortunato is drunk. Montresor has an easier time manipulating his victim because he is too intoxicated to be thinking clearly. So it is to Montresor's advantage to keep Fortunato drunk until he has him in chains. While they are in the catacombs Montresor twice gives Fortunato more wine to drink. He gives him the Medoc after Fortunato has a long coughing fit. The Medoc helps by relieving the cough, but it hurts by keeping Fortunato drunk and at the mercy of his crafty enemy.
Fortunato's cough is another clever invention of author Edgar Allan Poe. It takes a long time for Montresor and Fortunato to get from the streets to the catacombs. The fact that Fortunato has a cold and a bad cough are evidently intended to prevent him from doing much talking. Otherwise he might ask a lot of awkward questions about this cask of Amontillado. Since Montresor specifies that Fortunato is an expert judge of this Spanish sherry, Fortunato might ask questions which Montresor could not answer. There is a lot of casual chit-chat about coats of arms, Masons, and other things. The men have to be talking about something--but Poe doesn't want them talking about Amontillado!
The reader does not realize that the Amontillado is totally fictitious until Montresor has Fortunato chained to the rock wall of the catacombs. At this point Fortunato quickly sobers up. Montresor specified that the victim of the perfect revenge must know who is responsible for it and why. Poe keeps Fortunato drunk for as long as it is helpful to the plot, and then makes him sober up quickly when his sobriety is essential to the plot.
I had scarcely laid the first tier of the masonry when I discovered that the intoxication of Fortunato had in a great measure worn off. The earliest indication I had of this was a low moaning cry from the depth of the recess. It was not the cry of a drunken man.
When Fortunato cries, "For the love of God, Montresor!" this proves that the victim is sober and knows the identity of the man who is murdering him. It is the only time in the story that Fortunato calls Montresor by name. Montresor is satisfied that he has achieved the perfect revenge.