AIM: The aim of the study was to compare the dose‑adjusted plasma levels of carbamazepine (CBZ) among expressers and nonexpressers of cytochrome P450 3A5 (CYP3A5)*3 genotypes.SUBJECTS AND METHODS: The study was carried out in 100 epileptic patients who were on CBZ monotherapy. Steady‑state plasma CBZ levels were measured using reverse‑phase high‑performance liquid chromatography method, and genotyping of CYP3A5 was done using real‑time polymerase chain reaction method.RESULTS: Patients inheriting CYP3A5*3/*3 variant (nonexpressers) had an increased plasma concentration of CBZ (4.86 μg/ml) when compared to patients inheriting either CYP3A5*1/*1 or CYP3A5*1/*3 (expressers) (4.3 μg/ml, P = 0.004). Nonexpressers had significantly increased plasma concentrations of CBZ when adjusted for dose and weight when compared to expressers (P < 0.002 and P < 0.001, respectively). The frequency of adverse reactions in expressers and nonexpressers was 12% and 9%, respectively.CONCLUSION: There is a significant influence of CYP3A5*3 genetic polymorphism (6986A>G) on dose‑adjusted plasma levels of CBZ in epileptic patients in the South Indian population.