Swollen lymph nodes on the underside of the chin are mostly caused by bacterial infection, which manifests as inflammation of the lymph nodes. Treatment requires oral antibiotics such as cefixime, cefradine, and quinolones such as ciprofloxacin or levofloxacin, etc. The swollen lymph nodes can subside after about a week of treatment. It is also possible that the inflammation in the oral cavity may cause secondary enlargement of the lymph nodes under the chin. It is necessary to carefully examine the condition of the gums in the oral cavity, such as gingivitis or gum abscess, etc., and actively treat the original disease in the oral cavity by rinsing the mouth with a solution containing metronidazole.