Patients with one or two copies of the HLA-A*33:03 allele who are treated with allopurinol may have an increased risk of severe cutaneous adverse reactions (SCARs) as compared to patients with no HLA-A*33:03 alleles or negative for the HLA-A*33:03 test. This allele has been shown to be in linkage disequilibrium with the HLA-B*58:01 allele in some populations, which has a strong association with allopurinol-induced SCARs. Other genetic and clinical factors may also influence risk of allopurinol-induced adverse reactions.