Hi and welcome to HCM. Naproxen belongs to group of Nonsteroidal anti-inflammatory drugs. It is used to relieve pain from various conditions such as headaches, muscle aches, tendonitis, dental pain, and menstrual cramps. Common side effects of this drug are : Upset stomach, nausea, heartburn, headache, drowsiness, or dizziness may occur. Serious side effects (occurs rarely): easy bruising/bleeding, difficult/painful swallowing, hearing changes (such as ringing in the ears), mental/mood changes, swelling of the ankles/feet/hands, sudden/unexplained weight gain, change in the amount of urine, unexplained stiff neck, vision changes, unusual tiredness. Constipation, weakness and feeling sick could be due to some other reason, it needs to be evaluated. Please visit a local doctor for proper diagnosis and treatment. You are not suppose take this drug without a doctors prescription. If you are having fever paracetamol tablets are better than naproxen Regards Dr Santosh