The principal and potentially serious toxic effects of IMURAN are hematologic and gastrointestinal. The risks of secondary infection and malignancy are also significant (see WARNINGS). The frequency and severity of adverse reactions depend on the dose and duration of IMURAN as well as on the patient's underlying disease or concomitant therapies. The incidence of hematologic toxicities and neoplasia encountered in groups of renal homograft recipients is significantly higher than that in studies employing IMURAN for rheumatoid arthritis. The relative incidences in clinical studies are summarized below: 

Hematologic: Leukopenia and/or thrombocytopenia are dose-dependent and may occur late in the course of therapy with IMURAN. Dose reduction or temporary withdrawal may result in reversal of these toxicities. Infection may occur as a secondary manifestation of bone marrow suppression or leukopenia, but the incidence of infection in renal homotransplantation is 30 to 60 times that in rheumatoid arthritis. Anemias, including macrocytic anemia, and/or bleeding have been reported. 

TPMT genotyping or phenotyping can help identify patients with low or absent TPMT activity (homozygous for non-functional alleles) who are at increased risk for severe, life-threatening myelosuppression from IMURAN. See CLINICAL PHARMACOLOGY, WARNINGS and PRECAUTIONS:Laboratory Tests. Death associated with pancytopenia has been reported in patients with absent TPMT activity receiving azathioprine. 6, 20

Gastrointestinal: Nausea and vomiting may occur within the first few months of therapy with IMURAN, and occurred in approximately 12% of 676 rheumatoid arthritis patients. The frequency of gastric disturbance often can be reduced by administration of the drug in divided doses and/or after meals. However, in some patients, nausea and vomiting may be severe and may be accompanied by symptoms such as diarrhea, fever, malaise, and myalgias (see PRECAUTIONS). Vomiting with abdominal pain may occur rarely with a hypersensitivity pancreatitis. Hepatotoxicity manifest by elevation of serum alkaline phosphatase, bilirubin, and/or serum transaminases is known to occur following azathioprine use, primarily in allograft recipients. Hepatotoxicity has been uncommon (less than 1%) in rheumatoid arthritis patients. Hepatotoxicity following transplantation most often occurs within 6 months of transplantation and is generally reversible after interruption of IMURAN. A rare, but life-threatening hepatic veno-occlusive disease associated with chronic administration of azathioprine has been described in transplant patients and in one patient receiving IMURAN for panuveitis.21, 22, 23 Periodic measurement of serum transaminases, alkaline phosphatase, and bilirubin is indicated for early detection of hepatotoxicity. If hepatic veno-occlusive disease is clinically suspected, IMURAN should be permanently withdrawn. 

Others: Additional side effects of low frequency have been reported. These include skin rashes, alopecia, fever, arthralgias, diarrhea, steatorrhea, negative nitrogen balance, reversible interstitial pneumonitis, hepatosplenic T-cell lymphoma (see Warnings – Malignancy), and Sweet's Syndrome (acute febrile neutrophilic dermatosis).