Background: Colitis is an inflammatory bowel disease, which is treated effectively with antioxidant and anti-inflammatory drugs. This study evaluated the anti-inflammatory effects of the extracts of Viola odorata and Cassia fistula on the acetic acid-induced colitis in rats. 
Methods: We determined the total contents of phenols, alkaloids, saponins, and tannins in the plants’ extracts. Further, we used 28 male Wistar rats in four groups of seven each. Colitis was induced in the experimental groups by the intra-rectal administration of 1% acetic acid. Distilled water was used in the sham group. After induction of colitis, the control group received distilled water, the sham group received normal saline, the standard group received 360 mg/kg oral sulfasalazine, and the experimental group received the combined extracts at 200 mg/kg orally. The severity of colitis was assessed in all animal groups. 
Results: The phytochemical assays showed that both extracts contained alkaloid and saponin. Also, the V. odorata extract contained tannin while C. fistula had anthraquinone. Acetic acid increased the thickness of the colonic epithelial layer and caused edema, cell necrosis, and increased myeloperoxidase enzyme in the colon tissues. The inflammation, colon weight per unit area, and macroscopic scores in the group treated with the combined extracts were reduced more than that in the standard group. The extracts reduced the activity in the experimental group. However, sulfasalazine resulted in a better healing of the colitis.
Conclusion: The combined extracts at 200 mg/kg effectively reduced the colitis induced by acetic acid in the rats.