I think I read this sometime in the 1970s. The main character is a woman. As the title implies the dream snake is used as a, psychotropic. The user I think keeps it as a pet inducing the little snake to bite. The bite then results in an altered state of consciousness which then enhances the heroine's ability to succeed. I vaguely remember the snake may have been colored emerald and ruby. I remember it reminded me of the gentle tiny decay and ring necked snakes I came across as a boy who was always exploring in the woods of western Pennsylvania.
Could it be the Hugo- and Nebula-award winning 1978 novel entitled Dreamsnake by Vonda N. McIntyre?
The novel follows a healer on her quest to replace her "dreamsnake", a small snake whose venom is capable of inducing torpor and hallucinations in humans, akin to those produced by drugs such as LSD or heroin.
It was based on an earlier novellete, "Of Mist, and Grass, and Sand", in 1973 which also won a Nebula award, but probably wasn't published singly.