There are many causes of pain under the breast, which can be categorized into non-disease and benign and malignant diseases. 1. Non-disease causes: such as physiological breast hyperplasia, pregnancy, miscarriage, long-term use of drugs and so on. (1) Physiological breast hyperplasia: elevated estrogen before menstruation will lead to breast hyperplasia and pain when pressed. (2) Pregnancy: Pregnant women experience pain with pressure on the underside of the breasts throughout pregnancy. (3) Miscarriage: women after miscarriage will experience breast tenderness with pressure because of changes in progesterone. (4) Long-term use of drugs: some drugs that affect androgens in the body, such as isoniazid, metronidazole, methotrexate and so on, can lead to breast tenderness. 2. Benign and malignant diseases. (1) benign diseases such as mastitis, fibroadenoma and other benign tumors, mental factors such as anxiety, depression and so on. (2) Malignant tumors: breast cancer can cause pressure pain under the breast. There are many causes of pressure pain under the breast, we should seek medical treatment in time to clarify the cause of the disease and treat accordingly.