The common causes of pain under the left breast in men include the following: First, male breast enlargement: male breast enlargement is mainly caused by poor liver function, resulting in reduced estrogen inactivation. Generally, male breast enlargement usually requires further liver function testing, and oral Chinese medicine is needed to obtain clinical symptom relief when symptoms are severe. Second, intercostal neuritis, herpes zoster: you need to be alert to the possibility of intercostal neuritis or herpes zoster, and generally speaking, if it is herpes zoster, it usually needs to be treated with antiviral drugs. Therefore, if there is pain under the left breast in men, the most common cause is cystic hyperplasia of the breast, and it is necessary to perform a mammogram first to clarify the diagnosis and then treat the symptoms in order to obtain relief of clinical symptoms.