What can cause a canker sore on tonsil?

Canker sores typically do not occur on tonsils. Canker sores are small, round, shallow ulcers that form inside the mouth. They can be painful and may make eating and drinking difficult. Canker sores are not caused by a virus or infection, and they are not contagious. While canker sores can appear on the inside of your cheek, the inner lining of your lips, the base of your gums, the underside of your tongue, or the roof of your mouth, it's uncommon to have them on the tonsils. When there is a lesion on the tonsil, it's important to see a healthcare professional for proper diagnosis and treatment.