Side effects of glucosamine and chondroitin supplements Preliminary studies show glucosamine sulfate and chondroitin sulfate to be safe and well tolerated. Common side effects may include: Nausea Diarrhea Gastrointestinal upset These side effects are reported in a small portion of patients. However, even substances that are found naturally in the body can have unpredictable results when taken in higher than normal quantities and in different formulations. This is particularly true with patients who are taking multiple medications or who have other diagnosed illnesses. For example: Studies suggest that glucosamine sulfate may increase insulin resistance. For this reason, diabetic patients should use glucosamine sulfate with caution and only under medical supervision. Patients on blood thinners (anticoagulants) should use chondroitin sulfate only after discussing and receiving approval from their physician. By: Christopher D. Chaput, MD November 29, 2000