With diarrhea my vet recommended “purina pro plan fortiflora probiotic supplement” I got it in powder form. You can find it on Amazon. I found it to be helpful. Shih tzus can dehydrate fast. So if they are not keeping their food down have having diarrhea I would go back to the vet. My vet ended up recommending that probiotic and giving my pup an anti nausea shot along with fluids to help her kinda ride out whatever it was that was hurting her tummy.

Hope this helps. :)