Oh dear~ I just saw this...poor little guy! But it sounds like what your vet recommended will work. I just went thru something similar (not as bad and he's not a pup) with Madison.....the Pepcid works wonders I think and I'd been feeding him rice and ground turkey, so I'm continuing with half that and half of the food (W/D) my vet recommended. He's been amazingly well ever since, which makes my heart sing
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It's amazing how sick they can seem so fast and just a little thing really helps so much! Continued prayers for Pi'i for a complete and quick recovery!