Curcumin, a major constituent in rhizomes of Curcuma longa L., has shown various biological activities. It has widely been used as a food additive to provide potential health benefits. In the present study, we investigated changes in chemical stability and cytotoxic properties of curcumin and commonly consumed over-the-counter (OTC) drugs including ibuprofen, acetylsalicylic acid (Asp), and acetaminophen (AAP), through their interactions. Stability of curcumin was significantly improved in phosphate-buffered saline or 0.01 N HCl containing each OTC drug; Asp showed the most prominent effect. Stability of Asp or AAP during 24 h incubation with curcumin was not changed significantly. Cytotoxic effects of curcumin were enhanced in the presence of the OTC drugs on INT 407 normal intestinal and HCT 116 colon cancer cells. Relative cytotoxicity of curcumin (>10 μM) under the drug-treated conditions was significantly higher. Cellular uptake of curcumin in HCT 116 cells increased significantly when incubated with Asp or AAP. Intracellular thiol levels of the cells treated with curcumin were further reduced in the presence of the OTC drugs. The present study provides information that commonly consumed OTC drugs affect chemical stability of curcumin in physiological conditions, and certain bioactivities of curcumin can be altered through their interactions.