Azathioprine is an immune suppressive drug used in treatment of immune-mediated diseases in dogs. This medication can cause serious side effects such as bone marrow suppression, stomach upset, and liver toxicity. Frequent monitoring of your pet is needed at the beginning of therapy. Most dogs treated with this medication tolerate it well with no adverse events. However, if you note any vomiting, diarrhea, loss of appetite, weakness, bruising, or dark tarry stool please STOP medication and call our office immediately.