Tablet brittleness index (TBI) quantifies tablet fracture behavior, which strongly correlates with tablet porosity and tensile strength. The goal of this work was to quantify the influence of lubrication on tablet brittleness. Mag-nesium stearate was used as a lubricant. Several common tablet excipients, i.e., starch, lactose, microcrystalline cellulose (MCC), and dibasic calcium phosphate anhydrate (DCPA), and a binary mixture between lactose and MCC were studied. Tablet tensile strength (a) and TBI at zero porosity (sigma(0) and TBl(0)) were obtained from nonlin-ear regression of data of all powders to evaluate the relationship between brittleness and bonding strength of pore-free tablets. The results show that lubrication by magnesium stearate led to decrease in sigma(0) and increase in TBI0. The effect was more profound for both a longer blending time and a higher amount of magnesium stea-rate. In addition, the TBI of the binary mixture was successfully predicted from the fitted parameters of lactose and MCC using the power mixing rule. Such effects of lubrication on tablet brittleness should be considered dur-ing formulation development to avoid unexpected quality issues. (C) 2016 Elsevier B.V. All rights reserved.