If blood, plasma, milk or pus flows from the nipple during non-pregnancy, or if milk still flows after stopping breastfeeding for more than six months, it is called nipple overflow. Nipple overflow is one of the common symptoms of breast diseases. According to statistics, those who visit the doctor with nipple overflow as the first symptom account for 3% to 14% of breast diseases, and the incidence is second only to breast lumps and breast pain. Most of them are caused by physiological causes or benign lesions, and some are caused by malignant tumors. Bilateral nipple discharge may be physiologic, pharmacologic, systemic benign disease, or mastopexy. Physiological is common, not pathological, and is commonly associated with prolonged postpartum breastfeeding. A few women also experience short periods of overflow after intense orgasm due to high blood vessel congestion in the breasts, breast distention and nipple erection. Some menopausal women, due to endocrine disorders will secrete a small amount of breast milk. Taking estrogen or birth control pills can also cause nipple overflow. Bilateral nipple overflow can also be pathological, such as a condition called amenorrhea-overflow syndrome, which is caused by hypothalamic pituitary lesions and is accompanied by amenorrhea, headache, narrowing of the visual field, and elevated prolactin in the blood in addition to overflow, and can be diagnosed by CT or MRI examination of the brain. In patients with mastocytosis, the overflow is usually obvious before menstruation and disappears after menstruation. In those with unilateral nipple overflow, most of them are related to diseases of the breast itself. Common diseases include intraductal papilloma, breast hyperplasia, ductal dilatation of the breast and breast cancer. Unilateral hemorrhagic overflow from a single orifice should be further examined, and even more important if accompanied by a breast lump. Nipple discharge is an important breast symptom, of which 10% to 15% of unilateral discharge may be breast cancer. Regardless of the type and nature of the overflow, it should be taken seriously and a timely visit to the hospital should be made. Depending on the specific situation, ultrasound, mammogram, MRI and mammography should be chosen to clarify the diagnosis and deal with it as soon as possible. It should be noted that if nipple overflow occurs in men, it is more likely to be a malignant tumor of the breast and should not be taken lightly.