Prozac (fluoxetine) 40mg for headaches, back pain. wanted to try stepping back down to 20mg. dr. wants to keep me at 40mg. odd that doctor doesn't want to reduce?

Maybe: If symptoms were severe in past and too risky to lower dose, i could see his/her concerns. If no significant issues in past, i would favor lowering dose, or eventually tapering off and see how you are at a baseline.

Not really: I am not aware of any evidence that Prozac provides a benefit for pain. A class of medications called snri's is likely a better choice. Cymbalta (duloxetine) is now fda approved for musculoskeletal pain and it may be a better option. You should discuss this with your physician.