Naproxen is commonly used to reduce fever and to treat pain and inflammation caused by several conditions, including arthritis, gout, ankylosing spondylitis, tendinitis, bursitis, menstrual cramps, headache, muscle pain, back pain, and tooth pain. Naproxen may also be used for other conditions as determined by your healthcare provider. Naproxen works by blocking certain substances in the body that cause pain, inflammation, and fever.