Charlotte was given to me as a 6 pound 6 year old lady, and I've had a wild ride with her health.
At first things were good, but then she started getting diarrhea, vomiting, losing weight, not eating, and peeing outside the litter box. I took her to the vet where I spent $2000 on tests to no avail, so we went with a prescription food.
Even then, she wouldn't touch the prescription food that was to help her sensitive stomach. She kept meowing for food, but wouldn't touch what I gave her. She went down to 4 lbs, which was really concerning.
I tried so many things, until recently I decided to just put some canned chicken breast under her prescription food- my god it's working. She doesn't pee outside the litter box anymore (unless I forget to scoop it), and her weight is back up.
Honestly I just think she hated the food I gave her so much that she would rather die than eat it. I also noted she likes diversity in her food, so I swap between canned tuna and canned chicken mixed with her normal prescription food. She also is much, much happier.
Great news!