In summary, the CYP2B6 and CYP3A4 polymorphisms were analysed, for the first time, in HIV/TB co-infected Thai adults receiving efavirenz and nevirapine based-ART co-administered with rifampicin and the results indicated that only 516G>T in CYP2B6 gene, but not CYP3A4 gene polymorphism, gave the significant effects on plasma drug levels. The gene discussed is CYP2B6; the disease is tuberculosis.