Common side effects of mesalamine include stomach problems (such as burping, abdominal pain, constipation, and diarrhea); a headache or dizziness; nasal problems, such as nasal congestion, runny nose, sneezing, or itching; or a rash. For most people, these side effects lessen or go away after a few weeks of treatment. More serious side effects with mesalamine are less common or rare, but include allergic reactions, bloody diarrhea or blood in the urine, and an intolerance syndrome.