Sorry to hear your Shih Tzu is unwell, however I have worked out an answer for you below ...
Vomiting and diarrhoea are very common in the dog and this is often not such a serious situation. For example it can occur when a dog has eaten something she should not have out in the yard, or has had some unaccustomed rich foodstuff, or somehow acquired has a minor tummy bug of some kind. But of course this sort of thing is always a gamble so if you want to be 100% sure you are doing the right thing you should have a vet check her over without delay.
That said that there are a number of home remedies for vomiting and diarrhoea in the dog which some owners try before getting their vet involved, these can often work well for uncomplicated cases. Here are some examples ...
1. I am aware that some owners use Pepto-Bismol to treat their dogs with gastro-enteritis and report success. Here is a link to a web page which has all the information you might need including dose rates : LINK
2. Some owners try and control the situation with diet alone before they try anything else. This is logical and often works, the basis of this is to give the intestinal tract a complete rest and then bring it back slowly to function, resting the alimentary tract is the critical factor. It would go something like this ...
A. Starve her for 24 hours but make sure she has access to water.
B. The next day feed her four or five small light meals in the day. These meals should be the likes of chicken, fish, scrambled egg etc. with a little rice.
C. If she picks up on this diet gradually reintroduce her normal diet over the next few days.
3. Live yogurt added to the food or given orally can often help. Some gastro-enteritis situations are caused by an imbalance in the bacteria in your dog's gut. Live yogurt is full of " good " bacteria and will seed them into your dog's intestinal tract and restore the balance. a tablespoon twice a day would be fine for a dog of your one's size.
Of course if any of the above does not quickly start to work or if she deteriorates further in any way you should have your vet check her over just to be on the safe side.
I hope I have covered your question fully enough but if you would like further clarification or to talk things over a bit more then I will be on-line for the next hour or so and I will be more than pleased to continue working with you.
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