In clinical pharmacotherapy, therapeutic benefits and adverse effects of medicines differ substantially between individuals and are often determined by their blood levels. Critical regulators influencing the pharmacokinetics and pharmacodynamics of drugs include drug transporters and drug-metabolizing enzymes. Among these, we have focused on P-glycoprotein (P-gp), a drug efflux transporter. A growing body of evidence indicates that the expression and functional activity of P-gp are altered under several pathological conditions, by exposure to substrate drugs of P-gp, and by ingestion of certain foods. In this critical review, we discuss the mechanisms by which anticancer drugs, most of which are P-gp substrates, alter the expression and functional activity of P-gp in tumors and normal tissues after chronic treatment. Accumulating evidence shows that various transcription factors, in addition to epigenetic and post-translational factors, modulate P-gp expression, which alters the pharmacokinetics and pharmacological effects of drugs. Therefore, it is important to consider individual patients with regard to drug-taking history, as well as levels of P-gp expression and function, when providing clinical pharmacotherapy.