Most small canker sores heal on their own in a week or two. You may need medical help if you have multiple ulcers, very large ulcers, your pain is severe, your sores do not heal after three weeks or they spread, or you develop a fever. See your doctor or your dentist – there are many medications and treatments that can help. Your doctor or dentist can help you make sure your problem is actually a mouth ulcer, and not a tooth abscess or a rare form of oral cancer. You can get some pain medications over-the-counter, but others require a prescription. It’s best to ask your healthcare provider which medication is best for you to use. There are numerous pastes, creams, liquids, and gels on the market that can ease the pain and speed healing of mouth ulcers. Use a product containing one of the following ingredients:  Fluocinonide (Lidex, Vanos) Benzocaine (Anbesol, Kank-A, Zilactin-B, Orabase) Hydrogen peroxide (Orajel Antiseptic Mouth Sore Rinse, Peroxyl) If you have multiple ulcers, a mouth rinse may be easier than applying gel to each individual sore. Ask your doctor or dentist about getting a mouth rinse containing dexamethasone or lidocaine. These can both ease pain, and dexamethasone can also reduce inflammation. You may need oral medication if your ulcers don’t heal with other treatments. Talk to your doctor about your options, and let them know about your health history and any medications you take. Some medicines for other ailments can be used to treat stubborn mouth ulcers, such as sucralfate (Carafate) and colchicine. Your doctor may prescribe oral corticosteroids if you have severe ulcers that won’t heal. These can have many side effects and are usually a last resort. Alert your doctor if you're pregnant or breastfeeding or if you have an autoimmune disease; you might be better suited to another treatment. Your doctor may cauterize your ulcers with a chemical or instrument. This destroys the damaged tissue, and may relieve pain and speed healing. Ask your doctor or dentist if this option is right for...

TL;DR in Vietnamese:
Đến gặp bác sĩ nếu các vết loét to hoặc đau dữ dội. Đề nghị bác sĩ kê toa thuốc giảm đau tại chỗ. Hỏi bác sĩ về nước súc miệng chứa thuốc. Uống thuốc do bác sĩ kê toa nếu các vết loét không lành. Sử dụng phương pháp đốt. Uống thực phẩm bổ sung để ngăn ngừa tái phát.