For pain in the medial side of the left arm, first, you need to consider muscle and ligament damage to the medial side of the left arm to see if the pain in the left arm is caused by lifting heavy objects, falling, or overly pulling the arm muscles. If there is no injury, consider whether the specific location of the pain is in the forearm, upper arm, or elbow joint. If the pain location is in the forearm, consider whether there is carpal tunnel syndrome, which manifests as pain and numbness in the arm, wrist, and elbow, and can be relieved by flinging the arm or massaging the wrist or arm. If the pain is located in the elbow joint, you need to consider elbow arthritis or humeral medial epicondylitis and humeral epicondylitis, which can be basically judged according to the location of the pain. If the pain is in the upper arm, biceps tendonitis and frozen shoulder can be considered. A more easily overlooked disease is cervical spondylosis, which can present with numbness and painful discomfort in the arm. Regardless of which disease it is, it should be seen and treated promptly.