The fracture mechanics of acetylsalicylic acid (ASS) and lactose monohydrate (LM) were studied using three-point beam bending experiments and compared with conventional tabletting performance. ASS was found to have an unusual behaviour in terms of its Young's modulus and tensile strength when determined with beams of different porosities. The Young's modulus as a function of beam porosity showed two exponential parts separated by a constant region and the tensile strength as a function of the porosity followed a non-exponential law. Tabletting experiments revealed that ASS undergoes different deformation mechanisms at the different compaction pressures associated with the porosity ranges covering the different regions. The different deformation mechanisms might have caused different crack and flaw patterns or different crack lengths, in particular at the beam surfaces, which are under maximum tensile stress during the tests. The unusual findings were, however, not reflected in experiments to determine the critical stress intensity factor as a function of beam porosity, because here crack propagation is controlled via a notch introduced into the beams. In contrast to ASS, LM behaved like the majority of materials i.e. Young's modulus, tensile strength and critical stress intensity factor were found to relate to the beam porosity exponentially.