There are many reasons for a lump under the collarbone, such as subcutaneous cysts under the collarbone, lymphadenitis and breast cancer. 1. Subcutaneous cyst below the collarbone: the sebaceous gland secretion is too exuberant, when the hair follicle mouth area is blocked, with the increase of sebum, it will gradually form a cyst-like change, which will lead to the phenomenon of a lump below the collarbone. Surgery is usually used to remove the cyst. 2. Lymphadenitis: Inflammation of the lymph nodes below the clavicle can be caused by bacterial infection, thus leading to the phenomenon of a lump below the clavicle. Antibiotics such as penicillin and ceftriaxone can be prescribed. 3. Breast cancer: part of breast cancer can be caused by lymphatic metastasis, resulting in the phenomenon of a lump below the clavicle. Radiotherapy, chemotherapy and surgical resection can be chosen according to the specific situation. If there is a lump below the collarbone for unknown reasons, it is recommended to go to a regular hospital, consult a professional doctor, cooperate with the doctor to complete the relevant examinations, identify the cause, and actively deal with it.