Thiopurines are a class of immunosuppressant and anti-neoplastic agents. They are widely used in the treatment of inflammatory bowel disease, hematological malignancies, and autoimmune diseases, but can cause significant toxicity. Inherited gene mutations are now recognized as independent risk factors for severe adverse drug reactions to thiopurines even at tenfold dose reductions. We present a case of thiopurine toxicity resulting in severe myelosuppression, hepatotoxicity, and alopecia in an individual with homozygous *3/*3 loss-of-function alleles in the NUDT15 gene. Our case highlights important differences in gene mutation frequencies between races that can help guide pharmacogenomic testing.