Sores inside the mouth, specifically canker sores, can be caused by injury or stress, according to WebMD. Some people are susceptible to sores from eating foods that irritate the tissue of the mouth. These foods can be high in acids and include tomatoes, strawberries, pineapples and others.
Braces and badly fitted dentures can also irritate the inside of the mouth and raise canker sores, claims WebMD. In other patients, sores are caused by a weakened immune system and vitamin and mineral deficiencies. Celiac disease and other diseases of the GI tract can also cause canker sores in the mouth. In some cases, the causes of the sores are unknown.
Trench mouth is another condition that causes sores inside the mouth, according to Mayo Clinic. The sores are specifically found on the gums and are a rare but serious type of gingivitis, or gum inflammation. They are caused by poor dental hygiene. Fortunately, trench mouth is neither contagious nor cancerous.
Thrush is another condition in which sores appear in the mouth, says WebMD. They are caused by a fungus and resemble cottage cheese. They are usually found on the tongue, the inside of the cheeks, the roof of the mouth, the tonsils or the back of the throat. They may hurt and bleed when they are either rubbed or accidentally scraped while brushing the teeth.