Objectives: Previous studies have reported reduced tacrolimus dose-adjusted exposure in individuals expressing the CYP3A5* 1 allele (reference singlenucleotide polymorphism identification number 776746). However, results on patients from South America are scarce. The objective of this study was to investigate the influence of CYP3A5 and MDR1 allelic variants and their correlation on the pharmacokinetics of tacrolimus and a modified release formulation of tacrolimus in stable patients with liver transplant. Materials and Methods: This