This document discusses drug incompatibility, which occurs when a drug is mixed with other substances and undesirable reactions can occur, affecting safety and efficacy. There are two types of incompatibility: physicochemical, which are unintended in vitro interactions between drugs and other product components; and therapeutic, which are unintended in vivo interactions after administration. Specific examples of physicochemical incompatibilities that can occur include precipitation, complex formation, effects of excipients on drug stability, eutectic mixtures, and partitioning between phases.