Canker sores usually form on the tongue, cheeks or inside the lip, notes WebMD. Canker sores are usually yellow or pale and have a red clusters or outer rings. Canker sores can be quite painful, and they often last for 5 to 10 days. This type of sore tends to come back after it's healed. It is not associated with fever. The cause of canker sores is unknown, although poor nutrition, viruses, bacteria, sudden weight loss, stress and trauma to the mouth may play a role in their development. Some people develop canker sores after eating trigger foods, such as citrus fruit, chocolate, nuts, coffee or potatoes. Low levels of vitamin B12, hormonal changes and chewing or biting the inside of the cheek may also cause canker sores.
Cold sores or fever blisters are also painful, advises WebMD. They usually last for 7 to 10 days and may accompany symptoms of cold or flu. The herpes simplex type 1 virus causes cold sores. Cold sores are contagious from the initial rupture of the blister until healed.Learn more about Conditions & Diseases