Magnesium sulfate is widely used as a food additive and as an orally administered medication. The aim of this study was to evaluate the possible cytotoxicity of magnesium sulfate on AGS human gastric adenocarcinoma cells and gastric mucosa in mice. A trypan blue exclusion assay was used to determine the reduction in viability of AGS cells exposed to magnesium sulfate, and then effects on cell proliferation were quantified. The role of magnesium sulfate-mediated pro-inflammatory cytokine production in AGS cells was also investigated. mRNA expression for IL-1β, IL-6, IL-8, and TNF-α was determined by RT-PCR, and secretion of these cytokines was measured by ELISA. Immunohistochemical evaluation of IL-1β, IL-6, and TNF-α expression was conducted in mouse gastric mucosa. Addition of 3 to 50 mM magnesium sulfate to AGS cells inhibited both cell proliferation and cell viability in a dose-dependent manner. Magnesium sulfate had little effect on production of IL-1β or IL-6 but significantly inhibited production of IL-8. The animal model demonstrated that magnesium sulfate induced production of IL-1β, IL-6, and TNF-α. These preliminary data suggest that magnesium sulfate had a direct effect on the stomach and initiates cytotoxicity in moderate concentrations and time periods by inhibiting viability and proliferation of AGS cells and by regulating expression and/or release of pro-inflammatory cytokines.