Gauging the clinical significance of P-glycoprotein-mediated herb-drug interactions: comparative effects of St. Johnʼs wort, Echinacea, clarithromycin, and rifampin on digoxin pharmacokinetics - Gurley, B J; Swain, A; Williams, D K; Barone, G; Battu, S K (Mol Nutr Food Res, 2008)