Left breast water can be considered as physiological or pathological nipple overflow. Pathological overflow can be caused by breast hyperplasia, intraductal papilloma and so on. 1. Physiological factors: elevated hormone levels before menstruation will cause increased secretion from breast ducts and colorless and transparent nipple overflow, but it is usually bilateral and can be relieved, and when you find running water on the left breast, you need to carefully check whether there is overflow on the other side as well. 2. Breast Hyperplasia: It is due to the imbalance of endocrine hormone metabolism and the rise of estrogen level, which stimulates the mammary glands to undergo proliferative changes. The affected side of the breast may have pain, lumps or nodules, and some patients have milky white or yellowish nipple discharge. 3. Intraductal papilloma: It is a benign tumor of the breast, and patients may have bloody or yellow nipple discharge from the affected side of the breast. When abnormal secretion appears on one side of the chest, it is recommended to consult a doctor in time to clarify the cause of the disease and then target treatment.