Purpose: To evaluate the toxicity of the dried seaweed, Sargassum plagiophyllum, extract (SPE) pretreatment in constipated mice.Methods: The dried seaweed powder was mixed with distilled water and extracted by autoclave at 121°C. Antioxidant activity of the extract was determined by 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay. Human normal colon cells were pretreated with SPE at 0 - 100 μg/mL for 24 h before challenging them with 100 μM hydrogen peroxide (H2O2). Intracellular reactive oxygen species (ROS) were quantified using 2',7'- dichlorodihydrofluorescein diacetate (H2DCFDA). Male ICR mice were pretreated for 14 consecutive days with SPE at 100, 500 and 1,000 mg/kg or lactulose at 500 mg/kg. Body weight and food intake were recorded daily. Constipation was induced with loperamide on days 12, 13 and 14 and fecal pellets evacuated over a 4-hr period. The ileum, liver, kidney, and spleen were collected for histopathological examination.Results: The IC50 for the radical scavenging capacity of SPE was 343.90 ± 4.21 μg/mL compared to 14.14 ± 0.71 μg/mL for ascorbic acid. Pretreatment with SPE was significantly reduced ROS production in human normal colon cells. Oral administration of all doses of SPE and lactulose for 14 consecutive days had no effect on food intake or body weight when compared to the normal control group. Defecation was significantly more frequent in mice pretreated with SPE at 100 mg/kg than in the constipation control group. Histopathological examination of the ileum, liver, kidney and spleen of pretreated constipated mice revealed no toxic effect from either SPE or lactulose. On the other hand, the loss of mucus-producing cells in the ileum of constipated mice was significantly lower in mice pretreated with SPE.Conclusions: These findings support the safety of SPE supplementation and may broaden itsapplication in clinical fields as an alternative drug or supplement for constipation management.