Piperine, a major component of black pepper, is used as spice and nutrient enhancer. The purpose of the present study was to evaluate the effects of acute and prolonged piperine exposure on cellular P-gp expression and function in vitro and in vivo. Piperine at concentrations ranging from 10 to 100 µM, determined by MTT assay to be non-cytotoxic, was observed to inhibit P-gp mediated efflux transport of [3H]-digoxin across L-MDR1 and Caco-2 cell monolayers. The acute inhibitory effect was dependent on piperine concentration, with abolishment of [3H]-digoxin polarized transport attained at 50 µM of piperine. In contrast, prolonged (48 and 72 h) co-incubation of Caco-2 cell monolayers with piperine (50 and 100 µM) increased P-gp activity through an up-regulation of cellular P-gp protein and MDR1 mRNA levels. The up-regulated protein was functionally active, as demonstrated by a higher degree of [3H]-digoxin efflux across the cell monolayers, but the induction was readily reversed by the removal of the spice from the culture medium. Peroral administration of piperine at the dose of 112 µg/kg body weight/day to male Wistar rats for 14 consecutive days also led to increased intestinal P-gp levels. However, there was a concomitant reduction in the rodent liver P-gp although the kidney P-gp level was unaffected. Our data suggest that caution should be exercised when piperine is to be co-administered with drugs that are P-gp substrates, particularly for patients whose diet relies heavily on pepper.