A man is a heavy drinker, I recall the story says he drinks 12 or 20 beers a night and sends his son (nephew?) who is the story's narrator to buy then for him. After drinking a beer which is 'bad' he slowly starts transforming into a horrific monster.
I have the feeling it's a Stephen King story, certainly it has that feel. Can anyone tell me what it's called and what book it's in?
I think the story is "Gray Matter" By Stephen King. It was published in Night Shift in 1978.
From Wikipedia
The story, told from the perspective of an older "local" man, begins as he is sitting around at a convenience store with a group of his friends during a heavy snowstorm. A young boy runs in, deathly afraid. The men recognize him as the son of Richie Grenadine, a local man who was injured some time ago in a work accident, and was given lifetime workers' compensation. With no need to support himself, Richie became a recluse, rarely seen outside the confines of his apartment except to purchase the cheapest of beer, although lately, he had been sending his son out to purchase his beer for him.
After speaking privately with Richie's son, a group of men including the narrator and store owner Henry decide to take the beer to Richie personally. On their way, Henry relates some of the terrifying experiences the kid had told him — of how one day his father drank a "bad" can of beer, implied to carry a mutagen, and since then has been slowly transforming into an inhuman blob-like abomination that detests light and craves warm beer. Spying on him one night, the boy saw his father eat a dead cat, causing him to finally seek help.
Arriving at Richie's home, the men confront Richie from behind his closed door, demanding that he come out. The odor pouring out from behind the door convinces the group that Richie was eating more than dead cats, speculating that he may be responsible for a recent rash of missing people. The men are horrified when Richie opens the door and shambles out, resembling more fungus than man. The rest of the men run off as Henry stands his ground, firing his pistol at the creature. The story ends with the narrator recalling how his brief glimpse of the creature made him realize it was in the process of dividing in two, and calculating the exponential growth the creature is capable of, as they sit at the convenience store, waiting to find out whether Henry or the creature survived.
