Patients with one or two copies of the HLA-B*58:01 allele may have an increased risk of severe cutaneous adverse reactions, such as Stevens-Johnson Syndrome and Toxic Epidermal Necrolysis, when treated with allopurinol as compared to patients with no HLA-B*58:01 alleles or negative for the HLA-B*58:01 test. However, conflicting evidence has been reported. Other genetic and clinical factors may also influence the risk of allopurinol-induced adverse reactions.