if you specifically set out with the intent of writing a piece of prose about drugs, it will likely seem contrived and completely transparent to anyone that reads it. Try and keep an open mind, and while it's always good to draw on your own past or experiences when you're writing, I find it's much easier to write spontaneously and let ideas flow rather than sitting down with the express purpose of writing about a topic, or with a moral etc.
the other thing, is that if you take ideas from other people your piece probably won't be as natural or convincing as it could be if YOU had conceived it - writing isn't something that you can force yourself to do. I suggest reading a wide variety of books (don't be discriminate of time period or literary styles, everything and anything is good) and trying to find a) a writer's style that you enjoy or that you think works very well, and b) a few central ideas that provoke you into spontaneous thought when reading - for some reason, most of the stories I write begin with a metaphor or a few lines that I compose in my head, and then a story or series of vignettes constructed around them.
of course, it's not a good thing to mimic completely one of your favourite authors - certainly read and try and appreciate the way in which they use direct/indirect dialogue, metaphor, simile, rhythm, wordplay, imagery etc., their strengths and weaknesses, and incorporate these techniques in your writing - but in its essence creative writing is all about finding your own style and motivation.
Always play to your OWN strengths as a writer, too - if you can't write convincing, realistic dialogue...work around it with your narration. Aren't great at writing long winded, extended metaphors and unnecessarily long, verbose descriptions? Write sparse in a modernist/imagist style to greater effect...
anyway, basically, you'll never unlock your true potential if you have to rely on others for ideas

everyone has mental blanks. Don't worry - there will come a day when you'll sit down and just start writing about everything and nothing and something will hit you...just keep at it, and have fun! Writing is supposed to be enjoyable
edit: forgot to add in, if there's one thing that's EXTREMELY helpful in writing an extended piece, KEEP A JOURNAL! jot down some ideas or phrases or fragments if you think of something, put pen to paper and flow with your consciousness, find something, work with it!
also, READ OTHER SHORT STORIES! Purchase/borrow one of those anthologies of 'english short stories' or 'science fiction short stories' etc. if that's your thing, find an author you like, see if they have an anthology etc. and write some notes in your journal on techniques they use as your reading, or phrases you like, and so on...read critically, but remember it's all for fun
