During the last couple of decades, efflux transporters have received considerable attention due to their ability to alter, either beneficially or detrimentally the pharmacokinetic and pharmacodynamic for an administered drug. The expression of the energy dependent transporter, member of the ATP binding cassette (ABC) transporters superfamily, is not only limited to cancerous tissues, but is also expressed in different normal tissues and barriers such as the blood brain barrier and placenta. Furthermore, its unique distribution at the sites of absorption such as small intestine has been shown to greatly affect the bioavailability of the drug-substrates, and thus altering their effect. In addition, the striking overlap of substrates between P-glycoprotein (P-gp) and the phase I enzyme cytochrome P450 3A4 (CYP3A4) in addition to their coexistence at the same site has been shown to act synergistically to decrease oral drug bioavailability. Interestingly, the co-administration of a drug-substrate and an inhibitor of P-gp have been shown to increase the plasma concentration of the drug-substrates causing lethal toxicities that warrants critical evaluation of drugs as whether or not they could be substrates or inhibitors to P-gp. The availability of various in vitro cell culture models and in vivo knockout models of P-gp are currently serving the pharmaceutical sciences community to deliver safer drug use and lower risks of drug-drug interactions based on P-gp interactions. Therefore, the purpose of the current review is to summarize the current knowledge about the role of P-gp in determining drug ADME profile, and its role in drug-drug-interactions and their clinical implications.