Can I use treated wood to replace and/or supplement floor joists under my bathroom? I am repairing significant water damage and have removed portions of the subfloor.

Absolutely. I just sistered 4 termite eaten floor joists in a utility room. They can be a teeny bit larger than SYP due to the wetness.

As long as they are construction grade, the answer is yes.

There are some units that are not construction grade and should not be used. This is because of location and size of knots.

Check with lumber yard to find out.

Chuck