UK presumably?
You're unlikely to get much at all, do you have a history of back problems? It's much easier if you've got a history to lean on. Personally I think it's as much about the way you say what you say as the choice of words. You need to make them seem like you know what you're talking about, but because you've been prescribed before not because you're a drug seeker.
One thing that I've found useful is that if you're prescribed co-codamol, codeine ir or tramadol or something (which is the best you can hope for really) I've always been able to get it upgraded to DHC by just asking for it and saying codeine makes me sick. Wait until the script or meds is in your hand and then if you haven't discussed the specifics of what you were going to be given you can just glance at it and ask 'oh any chance I could have DHC instead these ones made me feel sick last time and DHC worked just as well but didn't make me sick'.