In our study, the obtained results demonstrate that CYP3A5*1/*3 and CYP3A5*1/*1 genotypes were significantly associated with infra-therapeutic Tac concentrations at 48 h and 7 days after Tac initiation, took longer to reach the therapeutic range, and had a higher risk of acute GVHD and renal toxicity. Here, CYP3A5 is linked to toxicity.