If your other cats ingest DERMA-IONX it will not hurt them. I prefer to add the homeopathic to canned food given to the intended recipient so that it increases the odds the right cat gets the whole dose. When you add it to the water you don't know how much your cat has gotten and you will throw some away when you change the water. Vetionx provides a free empty spray bottle with the order so that you can add some of the oral homeopathic to it and then spray some onto the affected spot if you choose to do so.
I would also give PROBIOTICS and OMEGA 3,6,9 to restore the gut and reduce inflammation.
Please let us know how you do.