Nipple discharge is a common symptom of breast disease and is overwhelmingly caused by benign breast disease. Most nipple discharge occurs in non-lactating women who inadvertently notice dark red or yellow spots on their underwear. The most common disease is intraductal papilloma, which is mostly unilateral and can be bloody or non-bloody. Hemorrhagic overflow is also more common in breast cancer, especially in intraductal papillary carcinoma. Thin, clear yellow or tan overflow is mostly caused by benign breast diseases, such as cystic hyperplasia of the breast, ductal dilatation of the breast and intraductal papilloma; plasma clear water-like overflow is often caused by tumors; purulent overflow is often unilateral in onset, mostly mucous green or milky yellow, and may carry blood, mostly seen in inflammatory breast diseases. Therefore, unilateral single hole overflow from the breast, especially bloody overflow, should be paid special attention and need to be treated in hospital in time. Smear cytology examination of the overflow, ultrasound examination, mammogram, ductoscopy or other special examinations can be performed to clarify the diagnosis. Generally bilateral nipple overflow is mainly due to elevated prolactin levels in the blood, which may be physiological, caused by taking certain medications or systemic diseases. Frequent local irritation of the nipple can also cause milk secretion; long-term use of certain sedative drugs such as chlorpromazine, fenadine, mydrium, etc., anti-hypertensive drugs such as reserpine, alpha-methyldopa, etc., contraceptive drugs and other drugs such as methotrexate can also cause milk secretion; pituitary tumors, overflow-amenorrhea syndrome, hypothalamic dysfunction also often cause bilateral nipple overflow. In conclusion, if nipple overflow occurs, regardless of the manner and nature of the overflow, it should be taken seriously because the vast majority of non-lactating nipple overflow is a manifestation of various breast diseases. In particular, if nipple discharge occurs in men, the possibility of malignant breast tumors is high and should not be taken lightly.