When I flush the bathroom commode, it backs up into the bathtub on the adjacent wall. What can I do?

Those two fixture come together into the same drain. You have a clog after they connect downstream. Rod that main via a clean out. If you see no clean out you can rod down the tub overflow.

Hi Nowrench2:

As Ballengerb1 said... these lines come together and the main line is clogged. I would actually have you rod out the main line using the cleanout if accessible from underneath or at the wall, or go down the 3" roof vent near this bathroom, if present, or finally, lift the toilet and snake the drain through the toilet drain.

I avoid snaking main drains down tub and sink drain lines when main lines are clogged JUST IN CASE the object blocking the line is large (paper towels, feminine napkins or diapers, for example) and could cause the snake to get tangled and snag up at the 1.5" pipe for the tub!

Use a medium electric cable machine (see picture)... can be rented at any tool rental store. Be sure to purchase some leather gloves and a pair of safety goggles... just to be on the safe side.

Let us know how you make out...

MARK