Drug interactions due to efflux transport inhibition at the blood-brain barrier (BBB) have been receiving increasing scrutiny because of the theoretical possibility of adverse central nervous system (CNS) effects identified in preclinical studies. In this review, evidence from pharmacokinetic, pharmacodynamic, imaging, pharmacogenetic, and pharmacovigilance studies, along with drug safety reports, is presented supporting a low probability of modulating transporters at the human BBB by currently marketed drugs.