Headache or abdominal pain may occur. If either of these effects lasts or gets worse, tell your doctor or pharmacist promptly. Remember that this medication has been prescribed because your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Rarely, mesalamine can worsen ulcerative colitis. Tell your doctor right away if your symptoms worsen after starting this medication (such as increased abdominal pain/cramping, bloody diarrhea). Tell your doctor right away if you have any serious side effects, including: nausea/vomiting that doesn't stop, signs of kidney problems (such as change in the amount of urine, blood in the urine), dark urine, severe stomach/abdominal pain, yellowing eyes/skin, chest pain, shortness of breath. A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction, including: fever, swollen lymph nodes, rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.

Mesalamine is very similar to balsalazide, olsalazine, and sulfasalazine. Do not use medications containing balsalazide, olsalazine, or sulfasalazine while using mesalamine. This medication may interfere with certain lab tests (such as urine normetanephrine levels), possibly causing false test results. Make sure lab personnel and all your doctors know you use this drug.