A common conclusion from these investigations is that intestinal P-gp blockage or absence increases the systemic availability of medicines that are P-gp substrates, which may result in toxicity or improved efficacy. This does not, however, imply that all P-gp substrates will have decreased oral absorption when a P-gp inhibitor is present. Numerous drugs that are P-gp substrates have a good bioavailability due to intestinal P-gp saturation at clinically relevant doses. Vinblastine, digoxin, ritonavir, etoposide, indinavir, and verapamil are a few examples of P-gp substrates with excellent oral availability .