Curcumin (CUR) is the major orange pigment of turmeric and believed to exert beneficial health effects in the gastrointestinal tract and numerous other organs after oral intake. However, an increasing number of animal and clinical studies show that the concentrations of CUR in blood plasma, urine, and peripheral tissues, if at all detectable, are extremely low even after large doses. The evidence and possible reasons for the very poor systemic bioavailablity of CUR after oral administration are discussed in this brief review. Major factors are the chemical instability of CUR at neutral and slightly alkaline pH, its susceptibility to autoxidation, its avid reductive and conjugative metabolism, and its poor permeation from the intestinal lumen to the portal blood. In view of the very low intestinal bioavailablity, it is difficult to attribute the putative effects observed in peripheral organs to CUR. Therefore, metabolites and/or degradation products of CUR should be taken into consideration as mediators of the pharmacological activity. © 2012 BioFactors, 39(1):14–20, 2013.