How can I get my doctor to prescribe me this?

I am no drug addict or junkie...just a younger guy with a lot of health problems and have done the research that even tho narcotic drugs are dangerous and can become abusive, if taken as directed and followed carefully can work better for you than non narcotic drugs. I felt weird the first time I asked my psychiatrist if he could switch me from Ativan to Xanax because I just figured he would think "Oh lord here comes another junkie" But he never did that and he listened to what I had to say and I have been anxiety and panic attack free for a long time because I had the courage to ask. I have recently had a lot of problem with my coughing and sinus problems and all that other doctor lingo even had to have a CT Scan of my chest...My doctor prescribed me antibiotics and a cough syrup called CRON DM and that has been like taking over the counter cough syrup and nothing has changed. I'm still coughing my lungs up to the point that my whole chest feels like its being stabbed. I have a doctors appointment in an hour and I want to ask for Promethazin vc codiene syrup because I've done a lot of research on it and it seems to work very well. I have no intentions of abusing anything and I don't even drink beer because I don't like feeling like I'm not myself. How do I go about asking my doctor for this without sounding as if I'm just searching for narcotic drugs?