My mom's girl Piper gets ULTRA Acidophilus from GNC mixed in with her food. I come in capsul form and disolves almost immediately. It promotes the health of the digestive tract. Piper used to throw upa lot and now she dosen't it has helped sort out her tummy.
My cat was diagnosed by 2 different vets with FIP when she was 3-4 months old. She was eating 4 cans of cat food a day because she wasn't retaining the nutrients. She was put on Selective care by Development (if I recall), Prozyme and Jarro-Dophilus and it worked great. Within 1 week she had no more loose stools. (on the advice of my vet). The Prozyme helped her retain the nutrients and the Jarro-Dophilus put good bacteria back into her stomach. I would highly recommend both. She is now 14 months and had her 1 year check last week and in perfect health. I now give her both supplements for maintenance twice a week.
My vet prescribed Prozyme for my elderly cat, and it helped her - she gained weight, and her fur improved a lot. You can get it from Doctors Foster & Smith if your vet is really expensive, or doesn't carry it.