Medications used in anesthesiology contain both pharmacologically active compounds and additional additives that are usually regarded as being pharmacologically inactive. These additives, called excipients, serve diverse functions. Despite being labeled inert, excipients are not necessarily benign substances. Anesthesiologists should have a clear understanding of their chemical properties and the potential for adverse reactions. This report catalogs the excipients found in drugs commonly used in anesthesiology, provides a brief description of their function, and documents examples from the literature regarding their adverse effects.