The main causes of redness, swelling and pain under the tongue include: lingual inflammation, ulcers on the base of the tongue, and cysts of the sublingual gland. 1. Tongue inflammation: It is usually caused by the lack of iron, vitamins and other nutrients, anemia, bacterial or viral infections. Patients usually show swelling of the tongue as well as burning pain. When the inflammation spreads to the underside of the tongue, the underside of the tongue becomes painful, red and swollen. 2. Base of tongue ulcers: can be caused by eating hard food, pathogen attack, wearing braces to damage the mucosa of the base of the tongue, resulting in congestion, edema, obvious burning pain and other symptoms of the patient’s lesion. Ulcers on the base of the tongue are generally self-limiting and usually heal gradually in 7-10 days. 3. Sublingual gland cysts: Sublingual gland cysts may be pseudocysts or true cysts. However, whether they are pseudocysts or true cysts, when the cysts have secondary infections or are large in size, they can cause redness, swelling and pain under the tongue. Therefore, it is recommended to consult a doctor for timely and targeted treatment of redness, swelling and pain under the tongue.