There are five main possibilities for left-sided chest pain: first, cardiac system diseases, such as coronary heart disease or myocardial infarction, manifested as pain in the left anterior thoracic region, and may be accompanied by posterior back radiating pain, with a history of hypertension or myocardial ischemia. Secondly, costochondritis, which occurs in the left 2nd-4th rib cartilage, is mostly aseptic inflammation, and the pain often increases after cold or exertion. Third, cystic hyperplasia of the breast, common in women of childbearing age, manifesting as breast pain and nodular changes in breast enlargement, and the pain is aggravated before menstruation and can be significantly relieved after menstruation. Fourthly, acute mastitis, which mostly occurs in lactating women, is characterized by red, swollen, hot and painful breasts, and may have fluctuating lump formation. Fifthly, breast cancer, which often has no obvious clinical manifestation in the early stage, may cause breast pain and discomfort when the disease progresses to advanced stage due to the cancer invading the skin ligaments or nerves. When the left side of the breast pain occurs, you should seek medical attention in time to avoid blind medication to avoid serious clinical consequences caused by the progress of the disease.