Stomach upset, nausea/vomiting, constipation, headache, or joint/muscle pain may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly.You may sometimes see whole or partial tablets/capsules in your stool. If this occurs frequently, tell your doctor. You may not be absorbing enough of the medication.Remember that your doctor has prescribed this medication because he or she 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.Infrequently, 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, fever).Tell your doctor right away if you have any serious side effects, including: change in the amount of urine, dark urine, persistent nausea/vomiting, 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: 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.In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.