What are 2 major side effects of Naproxen? Following 3 months of treatment with Naproxen, the patient's rheumatoid arthritis is improved. However, he now complains of significant heartburn and gastrointestinal pain that are not controlled by antacids. What is the mechanism of gastric disturbance caused by Naproxen? Given the patient's gastric sensitivity to Naproxen what would be an alternative NSAID for treating his rheumatoid arthritis?