It's possible it's just runoff from your experience with painkillers, but jumping from that to having dreams about heroin is pretty extreme in my opinion. On a side note, opium is fun once in a while
In any case I can't interperet your dreams for you, but it's also possible it's symbolic of some other aspect of your life something you feel like you can't rid yourself of, or something you're really missing (aside from drugs). I actually had reoccurring dreams at one point in my life about sitting down and smoking cigarettes even though I knew it was terrible for me, and that it was a descision I would regret later I sat in a corner and smoked an entire pack of cigarettes, one after another. I don't really smoke cigarettes even now, and at the time I had the dream I had never smoked a cigarette (or weed either for that matter). But I was having other things in my life that the idea of the dream fit into. I was totally in love with (and sleeping with) my best friend's boyfriend and even though I promised myself a hundred times I'd stop and I knew it was going to blow up in my face I kept doing it. Happy ending to the story, he left his other girlfriend for me and he and I have been going out for two and a half years now PLUS he was the one that smoked me out my first time.