TPMT and erythema multiforme: 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).