Occurrence of severe neutropenia was inversely associated with dose reduction in UGT1A1*28 homozygous carriers (ORx10 unit = 0.62, 95% CI: 0.27–1.40, p = 0.249) and UGT1A1 heterozygous or wild-type patients (ORx10 unit = 0.87, 95% CI: 0.59–1.28, p = 0.478). Here, UGT1A1 is linked to neutropenia.