Amorphous solid dispersion (ASD) formulations are increasingly used by the pharmaceutical industry to develop poorly water-soluble drugs into effective oral dosage forms. Examples include the antifungal drug itraconazole, the HIV protease inhibitor combination, lopinavir/ritonavir and the immunosuppressive, tacrolimus. There is potential for significant variation in bioavailability of ASD and thus heightened concern regarding the therapeutic efficacy as generic versions of these poorly water-soluble compounds become approved. The variation in bioavailability is to be expected because of our limited understanding of the precise physical chemistry of drug polymer amorphous solid dispersion formulations.