(1) A substance added to a drug product formulation that affects the action of the active ingredient in a predictable way. (2) immunology a vehicle used to enhance antigenicity; e.g., a suspension of minerals (alum, aluminum hydroxide, or phosphate) on which antigen is adsorbed; or water-in-oil emulsion in which antigen solution is emulsified in mineral oil (Freund incomplete adjuvant), sometimes with the inclusion of killed mycobacteria (Freund complete adjuvant) to enhance antigenicity further (inhibits degradation of antigen and/or causes influx of macrophages). (3) Additional therapy given to enhance or extend primary therapy's effect, as in chemotherapy's addition to a surgical regimen. (4) A treatment added to a curative treatment to prevent recurrence of clinical cancer from microscopic residual disease. [L. ad-juvo, pres. p. -juvans, to give aid to]