The causes of underarm darkening include physiological factors (such as local friction, endocrine abnormalities, etc.) and pathological factors (such as acanthosis nigricans, post-inflammatory hyperpigmentation, etc.). 1. Physiological factors: (1) Localized friction: If the underarms are subjected to friction for a long period of time, it can lead to the phenomenon of physiological darkening, usually not accompanied by other uncomfortable symptoms, without special treatment. (2) endocrine abnormalities: women’s pregnancy due to abnormal hormone levels in the body, the axillary melanocyte function is more hyperactive, which can lead to the phenomenon of darkening of the armpits, accompanied by darkening of the skin of the neck, the inguinal skin darkening phenomenon. 2. Pathologic factors: (1) acanthosis nigricans: black underarms may also belong to acanthosis nigricans, often due to heredity, obesity and other reasons, in addition, the surface skin will also appear proliferative changes. (2) Post-inflammatory hyperpigmentation: If there were inflammatory diseases in the armpits, such as ringworm, eczema, etc., and the pigmentation can be left behind after the cure, it needs to be considered as post-inflammatory hyperpigmentation. There are many reasons for the darkening of the armpits, it is recommended that timely medical treatment, a clear diagnosis under the guidance of a professional doctor to take standardized treatment, not private treatment, so as not to lead to adverse consequences.