- Gauging the clinical significance of P-glycoprotein-mediated herb-drug interactions: comparative effects of St. John's 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)