This episode was based on the "Hunger", the first issue of Hellblazer, the Vertigo comic book that spawned the series.
The bottle/vessel Constantine creates with the Ring of Solomon is an empty bottle of Christian Brothers brandy.
Actor/stuntman Lonnie R. Smith plays Dr. Alec Holland in a single episode in "Swamp Thing: The series (1990)". Holland is the man who becomes the heroic monster, Swamp Thing. Swamp Thing is another DC comic/Vertigo character who's allied with John Constantine.