In terms of genre this is contemporary fiction.
It has potential and there are some good moments and real rawness in places, but overall it didn't feel like a short story to me, more like an extract from a bigger piece. It might be because it felt quite distant in the sense that there is a lot of telling, stage directions and the use of "I". I know Harry did a blog a while back about the over-use of "I" in 1st POV.
For example, "As I pressed the plunger home, I felt the wave of narcosis suffuse my mind and body. All thoughts of sex and cute little tits were rinsed from my thoughts as I slipped into semi-consciousness, like a ship drifting into the mist." All the "I's" add to the telling effect, plus 'felt' is a filtering verb which again add distance and a layer between the MC and the reader. One way for example to make the prose tighter and more immersive: "The plunger hit home, and the wave of narcosis suffused my mind and body. All thoughts of sex and cute little tits rinsed from my thoughts as semi-consciousness unfurled, like mist around a drifting ship." In this example, I'm not told what the MC does and how he feels, I'm in here with him, I watch the plunger hits home, I am with the MC as he slips into semi-consciousness, etc...
Another example, "I went into the kitchen to find little Elsie still there gouching out. Sitting opposite her at the formica table, I pulled out the little wallet I carried, it contained a works and so on." Again this can be tightened by losing some of the stage directions and unnecessary explanation: "In the kitchen, Elsie was still gouched out. Sitting opposite her I dropped my gear wallet on the table." Because the MC has just asked if he can go do a hit, the 'in the kitchen' implies he's walked into the kitchen, and just the fact that 'Elsie was gouched out' implies that how the MC finds her. Also I'm trusting the reader that because it is a story about an addict they can work out what a gear wallet is.
Something else that can make the story a lot more vivid is using all the sense. This story relies main on sight, and some hearing (dialogue), but we are missing smells, hearing (noises), and touch. For example instead of telling us he has greasy hair, get him to rake his fingers through his hair and wipe the grease off on his jeans. Show us how horrible the place he lives in is by adding rank smells, etc...
With short stories, every word counts and needs to earn its place. This story could be edited and tightened a lot. Furthermore, with short stories it is as much about what's not being said or what's implied as what's on the page.
I hope this helps.