Pain in the left side of the abdomen when breathing deeply may be due to the following reasons: 1. Gastrointestinal problems, pain in the left side of the abdomen accompanied by nausea, vomiting or bloating, diarrhea, may be gastritis, colitis or mechanical intestinal obstruction, or may be caused by overeating, long-term irregular eating, smoking, alcoholism, etc.. It is necessary to keep a regular diet and must be light, with the drugs prescribed by the doctor for supplementary treatment, and if necessary, surgery may be required. 2, may be pain caused by muscle spasm in the chest wall, need to go to the hospital for timely medical examination, and then take drugs to relieve after diagnosis. 3, most of the pain caused by left pleurisy is due to lung infection, often manifested as cough, cough and fever, there are also It may be a problem with the respiratory tract, such as difficulty in breathing, etc., and requires a chest CT to confirm the diagnosis.