AbstractThein vivoeffects of berberine (BBR), the widely used bioactive herbal ingredient from many traditional Chinese medicinal herbs, on the pharmacokinetics of carbamazepine (CBZ, a substrate of CYP3A) and its metabolite carbamazepine 10,11‐epoxide (ECBZ), digoxin (DIG, a substrate of P‐gp) and cyclosporine A (CsA, a dual substrate of CYP3A and P‐gp) were evaluated in rats. After a 2‐week pretreatment with BBR, the pharmacokinetic parameters of i.g. administered CBZ and ECBZ were not significantly altered. The pharmacokinetics of i.v. administered DIG was not modified by single and 2‐week pretreatments with BBR, but a dose‐dependent increase inAUCandCmaxwas observed in the i.g. administered DIG parameters in rats. TheAUCs of DIG with BBR (30 mg/kg, 100 mg/kg) were 133%, 170% (single) and 123%, 169% (2‐week) of control, respectively. TheAUCandCmaxof i.g. administered CsA with a 2‐week pretreatment with BBR increased by 62% and 43% (BBR 30 mg/kg,p< 0.05), 96% and 60% (BBR 100 mg/kg,p< 0.01), compared with the control. In conclusion, berberine produced a dose‐dependent increased bioavailability of digoxin and cyclosporine A by inhibition of intestinal P‐gp. No significant changes in CYP3A activity by berberine were observed. Copyright © 2009 John Wiley & Sons, Ltd.