What are the side effects of naproxen? The most common side effects from naproxen are: rash, ringing in the ears, headaches, dizziness, drowsiness, abdominal pain, nausea, diarrhea, constipation, heartburn, fluid retention, and; shortness of breath. Other important side effects include: fluid retention, blood clots, heart attacks, hypertension, and; heart failure.