Cat peeing everywhere - Resolved!
Long Story Short: Two years of inappropriate urination was finally fixed with gabapentin and amitriptyline. I have my cat back (and my house doesn't smell like cat pee).
The Details: My 15-year-old cat started peeing everywhere but the litter box about two years ago. It started when we took in another cat. There were no fights, but my cat clearly felt invaded. She would pee on the dog's bed, the furniture, my bed (while I was sleeping in it!), rugs, any laundry left on the floor... it was horrible. We did all the things they say (added litter boxes, different litter, Feliway diffusers, enzyme cleaners). Nothing helped. We also ruled out any medical causes.
Our vet said the options were to get rid of the second cat or make my old girl a porch cat. We didn't want to do that. So she said we could try fluoxetine. We tried the transdermal and the liquid, because my cat can't do pills. No change in peeing.
Finally, I went to a different vet for a second opinion. She recommended gabapentin (liquid, 50 mg, mixed into her food) and transdermal amitriptyline (rubbed on her ear). After a few days, she started peeing on the disposable pee pads in front of the litter box. I thought, this is great - if she does this the rest of her life, that's a win. After about a month, she actually started to PEE IN THE LITTER BOX again.
We are now a few months into this new phase. She is consistently peeing in the litter box. About once a week, she'll pee on the pee pad in front of the litter box, and that correlates to days where she did not eat all of her gabapentin-laced food.
And she is HAPPY! She is purring and affectionate like she used to be.
I'm just so happy my cat is able to relax in her own home, and can hopefully live out her years in peace. When we were going through this, I read every Reddit thread I could find about inappropriate urination, and as you know if you've dealt with it, often there's no resolution. Cats are finicky beings. I hope this helps someone.