1. Known history of allergic reaction or clinically significant intolerance to aspirin or salicylate derivatives (including mesalamine) or non-active ingredients of the investigational product 2. Onset of UP relapse >6 weeks prior to the Baseline Visit for patients experiencing a relapse of their UP (i.e., patients who are not newly diagnosed) 3. Severe UP as defined by a DAI score of greater than or equal to 11 or a PGA sub-score of 3 4. Histological Disease Activity Score > or equal to 1 for the biopsy taken from the normal tissue above the disease margin during the flexible sigmoidoscopy/colonoscopy performed within 14 days of the Baseline Visit 5. UP with disease involvement greater than 15 cm beyond the anal verge as confirmed on flexible sigmoidoscopy/colonoscopy 6. Prior unsuccessful treatment of active UP or active ulcerative colitis with rectally administered mesalamine preparations of any strength 7. Any prior treatment of UP or ulcerative colitis with any oral 5-aminosalicylic acid product if used at >2 g/day, regardless of treatment outcome 8. Use of local, rectally administered therapies for UP or ulcerative colitis (e.g., suppositories or enemas containing mesalamine, etc.) within 30 days of the Baseline Visit 9. Use of any of the following medications: - Biological therapies (e.g., infliximab) within 90 days of the Baseline Visit - Immunosuppressive/immunomodulating (e.g., azathioprine) medications within 90 days of the Baseline Visit - Oral, intravenous, intramuscular, or rectally administered corticosteroids within 30 days of the Baseline Visit; the use of intranasal and/or inhaled corticosteroids is permitted - Oral 5-aminosalicylic acid products within 7 days of the Baseline Visit, if used at & less than or equal to 2 g/day - Oral, intravenous, or intramuscular antibiotics within 7 days of the Baseline Visit - Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) within 7 days of the Baseline Visit; low-dose aspirin (less than or equal to 325 mg/day) taken for cardio-protective reasons is permitted - Antidiarrheals, antispasmodics, and iron therapy within 7 days of the Baseline Visit - Transdermal nicotine products within 7 days of the Baseline Visit 10. A change in regimen (i.e., dosage or frequency of use) of permitted medications within 30 days of the Baseline Visit, or any plans to change the regimen during the course of this study 11. Use or treatment with an investigational drug, therapy, or device within 30 days of the Baseline Visit 12. A planned change in tobacco usage (e.g., smoking, oral tobacco) during the study 13. Female patients who are pregnant, planning a pregnancy, or who are breastfeeding 14. Diseases interfering with the DAI assessment, including but not limited to, hemorrhoids and anal fissures 15. History of Crohn Disease, short bowel syndrome, or bowel surgery (except appendectomy), or active peptic ulcer 16. A positive stool culture for enteric pathogens (Salmonella, Shigella, Yersinia, Campylobacter, Vibrio, E. coli O157/H7), detection of Clostridium difficile toxin through immunoassay, or enteric parasites and their ova (including Giardia, Cryptosporidium, and Entamoeba histolytica) on routine microscopy at the Screening Visit 17. Significant impairment of renal or hepatic function, as defined by any of the following: - Creatinine >1.5 x Upper Limit Normal (ULN) - Alanine Amino Transferase (ALT) >2.5 x ULN - Aspartate Amino Transferase (AST) >2.5 x ULN 18. Serologic positivity for the Hepatitis B virus (HBV), the Hepatitis C virus (HCV), the Human Immunodeficiency Virus (HIV), or Treponema pallidum (the causative agent of syphilis) 19. Known history of idiopathic / chronic pancreatitis 20. History of active drug or alcohol abuse within the past year, or physical examination findings indicating the same 21. Current clinically significant urinary tract obstruction 22. History of coagulation disorders, including those requiring treatment with anticoagulant drugs (except for aspirin taken at ≤325 mg/day for cardio-protective reasons 23. Current active malignancy or history of malignancy within the past five years, except for cervical carcinoma in situ, squamous or basal cell carcinoma of the skin that has been surgically removed, or prostate cancer that is being managed by watchful waiting (observation alone) 24. History of pelvic irradiation 25. Any other clinically significant abnormal medical condition that in the Investigators judgment would put the patient at increased risk of illness or injury, would interfere with study participation or would interfere with the evaluation or quality of the data 26. Inability or unwillingness to understand and comply with the requirements of the protocol for any reason, including dosing procedures and visit requirements 27. Previous randomization in this study