Azathioprine is a cytotoxic drug which affects every dog in a different way. You can't predict the dose that will work, or that is toxic, just by weighing the patient. The most effective dose of azathioprine is the dose that reduces the neutrophil count or platelet count to the low normal range. So some dogs will need a dose reduction because their platelet count or neutrophil count gets too low. We have heard of German Sherpherds that have not responded - but these were generaly LARGE dogs (bigger than the small number we treated) - and these dogs probably needed more than 50 mg of azathioprine per day. Thus the starting dose of azathioprine is 50 mg per metre square surface area.....but you need to do a full bllood count to check the dose is not too high.....or too low.
Good luck with your dog, and your research!!