Many mouth sores are caused by bacterial, viral or fungal infections. These may result from a disease, disorder or chronic irritation. In other cases, the exact cause of the sores is not known. The most common causes of mouth sores include:

• Herpes simplex viruses. These viruses – primarily herpes simplex virus type 1, also known as HSV1 – cause cold sores and can remain in the body for a lifetime, reappearing during periods of illness, stress, trauma (including dental treatment) or sun exposure. Most people have been exposed to HSV1 at some point by the time of adulthood. Cold sores may occasionally be caused by HSV type 2, the form of herpes simplex that most commonly causes genital blisters, during oral to genital sexual contact.
  
  
• Unknown. For example, the cause of canker sores is unclear, although various factors may play a role in producing these types of mouth sores. These factors include tissue injury or trauma, immune system weakness (e.g., during a cold or the flu), genetics, allergies, bacteria or viruses, stress, hormones (e.g., during menstrual periods) and vitamin deficiency. People with ulcerative colitis or Crohn’s disease also appear to have an increased risk of developing canker sores.
  
  
• Fungi. A yeast infection known as thrush can cause mouth sores. It occurs due to an overgrowth of the Candida albicans fungus inside the mouth. The infection is also sometimes referred to as Candidiasis and is common in babies and people with compromised immune systems (e.g., people with diabetes, HIV/AIDS).
  
  
• Leukoplakia. A condition that is usually caused by chronic irritation of the mucous membranes lining the mouth (e.g., due to tobacco and/or alcohol use, ill-fitting dentures). It may also be caused by a fungal infection. Sometimes the cause of the condition is unknown. Leukoplakia causes skin discoloration inside the mouth.

The mouth can become irritated by a number of factors, which can lead to mouth sores. Common causes of mouth irritation include:

• Biting the cheek, tongue or lip
• Tobacco (e.g., cigarettes, cigars, chewing tobacco)
• Braces
• Dentures that are new or do not fit properly
• Sharp or broken teeth
• Burns from hot foods or liquids

Less common possible causes of mouth sores include allergies, cancer, autoimmune diseases (in which the body’s immune system attacks its own tissues and organs – such as rheumatoid arthritis or lupus) and comprised immune systems (e.g., human immunodeficiency virus [HIV] or acquired immunodeficiency syndrome [AIDS]).

Mouth sores can also appear as a side-effect of various medications. These include:

• Chemotherapy drugs
• Steroids
• Antibiotics (e.g., penicillin, streptomycin)
• Aspirin
• Barbiturates (used to treat insomnia)
• Anticonvulsants (e.g., phenytoin)