Allopurinal, a drug often given in conjunction with azathioprine or 6MP to lessen their side effects, can on rare occasions itself produce significant side effects. This research shows that African Americans, Asians, and Native Hawaiians are 3 to 7 times more likely to suffer severe skin side effects from allopurinol as compared to whites or Hispanics.
http://ard.bmj.com/content/early/2018/04/13/annrheumdis-2017-212905
http://ard.bmj.com/content/early/2018/04/13/annrheumdis-2017-212905