If you've got any galvanized drainage pipes, you should consider budgeting to replace them. While cast iron generally lasts, galvanized can have issues, and when they start, the rusting starts to make the surface very rough which traps hair and other debris that tends to lead to clogs, holding moisture making it rust faster. Adding something like Draino will cause the corrosion to speed up even more. You might get some relief with a bio treatment like BIO-CLEAN, but that's no panacea...replacing the pipe with something new and smooth inside is the long-term solution.
Galvanized has no real place in the potable water supply, either. IT will end up rusting out, too, especially on the hot lines. If you have it in the drain lines, there's probably some in the water supply lines, too. A disaster waiting to happen.