Are Canker Sores Contagious?

  • Medical Author:
    Charles Patrick Davis, MD, PhD

    Dr. Charles "Pat" Davis, MD, PhD, is a board certified Emergency Medicine doctor who currently practices as a consultant and staff member for hospitals. He has a PhD in Microbiology (UT at Austin), and the MD (Univ. Texas Medical Branch, Galveston). He is a Clinical Professor (retired) in the Division of Emergency Medicine, UT Health Science Center at San Antonio, and has been the Chief of Emergency Medicine at UT Medical Branch and at UTHSCSA with over 250 publications.

  • Medical Editor: William C. Shiel Jr., MD, FACP, FACR
    William C. Shiel Jr., MD, FACP, FACR

    William C. Shiel Jr., MD, FACP, FACR

    Dr. Shiel received a Bachelor of Science degree with honors from the University of Notre Dame. There he was involved in research in radiation biology and received the Huisking Scholarship. After graduating from St. Louis University School of Medicine, he completed his Internal Medicine residency and Rheumatology fellowship at the University of California, Irvine. He is board-certified in Internal Medicine and Rheumatology.

What are canker sores?

Canker sores are small painful ulcers in the mucous membranes that form the inner lining of the mouth. The cause of canker sores is unclear; no viral or bacterial cause has been proven. Researchers think that they occur or are triggered to occur in individuals when a certain set of circumstances arise such as

  • emotional stress,
  • hormonal changes,
  • allergies,
  • certain diseases (including Crohn's disease and celiac disease),
  • a lack of certain nutrients and vitamins,
  • sensitivity to certain chemicals in foods or drinks, or
  • even minor trauma while tooth brushing or other minor mouth trauma.

Canker sores can recur. Canker sores are sometimes confused with cold sores, but cold sores are due to herpes viruses and are contagious, while canker sores are not contagious and usually do not involve the lips.

Canker sores also are referred to as

  • aphthous ulcers,
  • aphthous stomatitis,
  • recurrent aphthous stomatitis, and
  • ulcerative stomatitis.

Are canker sores contagious?

Canker sores are not contagious. There is no person-to-person spread. Canker sores seem to develop in an individual when certain triggers occur (such as hormonal shifts, stress or exposure to certain foods or chemicals). Individuals that develop canker sores may not have the same triggers that cause canker sores to develop in other people.

How will I know if I have a canker sore?

Shallow painful ulcers with a reddish border are characteristic of canker sores. They may appear on the soft palate, tonsil areas, tongue, and gums of the mouth. There are three types of canker sores: minor, major, and herpetiform.

  1. Minor canker sores are usually smaller than 1 cm in diameter and account for about 80% of all canker sores.
  2. Major canker sores are greater than 1 cm in diameter, usually have irregular borders, and when they heal, the ulcers can leave scars and even distort the shape of the mucosal surface.
  3. Herpetiform canker sores are small - no larger than 1 mm - and usually occur in clusters (about 10–100 ulcers). Herpetiform canker sores are poorly named as they do not contain herpes viruses.

How do canker sores spread?

Canker sores do not spread from person-to-person. Canker sores are recurrent and may seem to spread in an individual if the mechanisms that trigger canker sore development recurs frequently in an individual.

When will I know I’m cured of canker sores?

There is no known cure for canker sores. There is only treatment for the painful symptoms. However, most minor canker sore problems last about 7 to 10 days while major canker sores may last weeks to months and may cause scars.

When should I contact a doctor about canker sores?

Most minor canker sores do not require a doctor to treat them. However large canker sores (major canker sores) should be seen by a health-care professional. In addition, rapidly reoccurring sores (new canker sores occurring before older sores heal), sores that persist longer than two weeks, sores that cause problems with either eating or drinking, and sores associated with high fevers should be seen urgently by a physician.


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Medically Reviewed on 11/9/2017
Mirowski, GW., MD. "Aphthous stomatitis." Medscape. UPdated: Mar 21, 2017.