Naproxen can cause side effects, but these won't affect everyone who takes it. The most common side effects of naproxen include confusion, headache, ringing in the ears, changes in vision, tiredness, drowsiness, dizziness and rashes. If you experience any signs of an allergic reaction, chest pains, or a high temperature with headache, sickness, sensitivity to light and neck stiffness, you must seek emergency medical attention. For more information on potential side effects of naproxen, refer to the patient information leaflet.