If your Shih Tzu is truly allergic to chicken then the prescription diet could be problematic and based on what Dr. Ken Tudor wrote in this PetMD article the diet is not necessary either.
“The solution to pollution is dilution” is the phrase we veterinarians now use to explain how to prevent urinary crystal and stone formation. Time, observation, and studies have shown us that there are no magical diets for solving this problem and that water consumption is key.
The take home for those of you with cats and dogs that are urine crystal formers is to increase the amount of water in the diet.” http://www.petmd.com/blogs/thedailyvet/ken-tudor/2015/november/water-solution-urinary-crystals-pets-33270
