I have two Shih Tzu and one has chronic IBS. I have tried pills, diet vet after vet. The most effective solution is a high protein dog food (all natural products and no by products) along with one can of tuna and sweet potatoes mashed together. She also gets one probiotic capsule with yogurt.
In her water I put one teaspoon of apple cider vinegar. So far this is the only thing that has worked. She does have the odd flare up but for the most part she is much better.