I do want to say on this site we are NOT Vets, if you wish to post this on another site where there is a Vet you may have to wait a few hours(days) for a reply but it would be much more indepth that what we can tell you here, all I can give is my own opinion.
here is a site to read more about cerenia, I see it is only approved for use on dogs, however I do know many drugs are used 'off label' for us as well as animals.
dragonlady....all I can say is if you don't feel comfortable with the drugs given and don't know why they were prescribed than I would d/c use until you could get an answer. IMO the dosage was very HIGH for a cat. and IMO any steroid should only be used if absolutely the last resort b/c of so many complications they can cause.
How is Shadow now? has the treatment helped his diarrhea?
Did the Vet have a diagnosis or was he just giving all this as a 'watch and see what happens' approach?
You mentioned the injections at the clinic but didn't say what you are administering at home??? what med is this?