Here, our findings showed a case of a patient with NSCLC, using gefitinib, doxycycline, and metamizole, plus homozygosity in ABCB1 rs1128503 (c.1236A>G), heterozygosity in ABCG2 rs2231142 and rs2622604 (c.421G>T+c.-20+614T>C), and the presence of a non-functional component in the CYP3A5 variant, and the presence of moderate to severe adverse reactions associated with the use of gefitinib. The gene discussed is CYP3A5; the disease is non-small cell lung carcinoma.