See also Warning section. Upset stomach, nausea, heartburn, diarrhea, constipation, headache, tiredness, drowsiness, and dizziness may occur. If any of these effects persist or worsen, notify your doctor or pharmacist promptly. If your doctor has directed you to use this medication, remember that he or she has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects. Tell your doctor immediately if any of these unlikely but serious side effects occur: stomach pain, difficult/painful swallowing, swelling of the hands/feet, sudden/unexplained weight gain, vision changes, hearing changes (e.g., ringing in the ears), mental/mood changes (e.g., depression), fast/pounding heartbeat, persistent/severe headache, fainting. Tell your doctor immediately if any of these rare but very serious side effects occur: change in the amount of urine, easy bruising/bleeding, signs of infection (e.g., fever, persistent sore throat), unexplained stiff neck. This drug may rarely cause serious (possibly fatal) liver disease. If you notice any of the following rare but very serious side effects, stop taking naproxen and tell your doctor immediately: yellowing eyes/skin, dark urine, unusual/extreme tiredness, severe stomach/abdominal pain, persistent nausea/vomiting. A very serious allergic reaction to this drug is unlikely, but seek immediate medical attention if it occurs. Symptoms of a serious allergic reaction may include: rash, itching/swelling (especially of the face/tongue/throat), severe dizziness, trouble breathing. This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist. In the US - Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088. In Canada - Call your doctor for medical advice about side effects. You may report side effects to Health Canada at 1-866-234-2345.