I am a disabled former paramedic, and as such I did a lot of research on meds and their effect on driving. Here is what I found:
There have been studies done on driving and people on narcotic pain killers. One study was done by the university of Chicago, and another was done in Finland. Both involved cancer patients on relatively large does of heavy duty meds (200+ mg of morphine per day) - much more than I take. What both studies showed is that after the initial two weeks the body adjusts to the does and their performance on driving tests is unaffected. The key is using as little as possible and maintaining a steady dosage.
In recent years we finally have large number of docs who specialize in pain control - and they have been a huge help to many people, including myself. They have a lot of tools besides narcotics and I would advise anyone in significant pain to see a pain specialist. The most effective thing I have isn't meds, it's a nerve stimulator implanted in my spine. It does a nice job of jamming the pain and has made the difference between being home bound or active.
Hope this helps someone.