Stop Using Litter Box

Question
I have a cat who is 15. I have another cat that is 4 and she was stray.  For the first year that we had the stray she used the litter box fine. So for the last couple of years the stray is peeing all over the house.  She will not pee in the litter box.  I have brought her to the vet serval times, tried seperate litter boxes, tried sprays and solutions.  I just don't know what to do?  Do you have any suggestions to help me?

Answer
You seem to have already done everything I would have suggested. Honestly, I thought it would be the older cat having litterbox issues; I can empathize with how frustrating it must be. Have you tried a different type of litter, or placed the litterboxes in separate rooms? Has the vet done tests to check for conditions like thyroid or diabetes (I'm not a vet, so I'm guessing at the types of tests they could do to check for metabolic issues rather than parasites).

In a pinch, perhaps the vet can even prescribe some sort of med like Elivil, which is actually used to ease up on anxiety (sort of like kitty prozac). It's a longshot, but it might help a bit.

I wish I had better answers. Let me know how things turn out.