It really depends a great deal on what kind of medication you are taking. NSAIDs like ibuprofen have very little impact on the central nervous system, and although they may have some slight effect on blood clotting, that isn't usually an issue for diving unless you get injured.
Narcotics, on the other hand, all have some degree of sedation as a side effect. This can be additive with the CNS effects of nitrogen narcosis. Although you may have habituated to your medications, that is still a concern. At the very least, you should be quite conservative about your depths, until you have a handle on how narcosis and your medications are going to mesh.
It is also a worry that your back problems are sufficiently severe to require routine narcotic medication. Are you going to have sufficient mobility to handle and move with gear? Are you going to have any issues getting on and off a boat? These are also things to think about.
