Pain in the left side of the neck when swallowing may be caused by the following reasons: First, inflammation: 1. Inflammation of the left side of the throat: the most common is viral infection or bacterial infection, resulting in inflammatory pain in the throat. 2. Inflammation of the left tonsil: swallowing pain in the left side of the neck can occur, and you can go to the ENT department of the hospital to make a clear diagnosis. Second, the left side of the neck esophagus is damaged: for example, eating hard things and damage to the esophageal mucosa, or stuck fish bones, foreign bodies and so on. Thirdly, lesions on the left side of the thyroid gland: the common ones are thyroid nodules, thyroid adenomas, and even thyroid cancer, etc. You can go to the hospital to take an ultrasound of the thyroid gland for diagnosis and differential diagnosis.