While azathioprine may offer some benefit, long-term use may lead to myelosuppression (especially in patients with low thiopurine methyltransferase level), hepatotoxicity, gastric irritation, and hypersensitivity syndromes (urticaria, morbilliform eruption, purpura, erythema multiforme, and angioedema). This evidence concerns the gene TPMT and urticaria.