The source of the pain determines what strains are most effective against it.
Sativas are generally used for nerve pain or inflammation, and indicas are effective against muscle aches and stress-related back/neck problems.

Furthermore, Cannabis in general has long been known to be very useful in counteracting the effects of opiate withdrawl and allowing people to 'get clean' off strong perscription meds that are harming them more than they are helping.

Best of luck to you.