What is the reason for on and off sharp stabbing left chest pain persisting for three years?

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Such stabbing pain is rarely cardiac, especially in youngsters. Most probably, this pain is psychogenic. Another common causes are precordial catch, in which sudden sharp localized chest pain occurs while breathing in and lasts for a few minutes at most. Costochondritis is another cause in which inflammation of rib joints causes pain., and tenderness is usually present. I would suggest muscle relaxants and painkillers for treatment. Further workup will be needed if associated with fever, cough, or real shortness of breath. Regards.