Canker Sores – Causes, Signs and symptoms

Canker Sores

Also called: Aphthous Stomatitis, Recurrent Mouth Ulcers, Aphthous Ulcers

Reviewed By:
Andrew M. Sicklick, D.D.S.


Canker sores are open lesions (ulcers) that appear inside the mouth. They are round, shallow and small (usually less than 1 centimeter in diameter) and tend to occur on the inner cheeks, inside the lip, at the base of the gums or underneath the tongue. They are usually preceded by a tingling or burning sensation, and typically appear as a pale-colored crater with a red border.

Canker sores are different from, but may be confused with, cold sores – contagious blisters that appear outside the mouth (usually around the lip area) that burst and crust over.

The cause of canker sores is unknown, but is believed to be related to dysfunction of a person’s immune system. Certain factors appear to make a person more susceptible to canker sores (e.g., gender, age, family history of canker sores). Conditions that may trigger an outbreak of canker sores include injury to the tissue lining of the mouth, stress, allergies and vitamin deficiency. Certain diseases (e.g., cancer, digestive disorders, HIV/AIDS) may also be associated with canker sores.

Patients who experience canker sores that are very large, extremely painful, recur frequently or have not healed after two weeks should seek medical attention. 

Canker sores can usually be diagnosed by visual inspection of the affected area by a dentist or other health care professional. A medical history may be taken to identify potential causes of the sores. Occasionally, additional tests (e.g., blood tests, biopsy) may be performed in order to rule out certain conditions or diseases.

In most cases, canker sores heal without treatment within approximately two weeks. There is no known cure for canker sores. Treatment remedies focus on alleviating a patient’s symptoms. These remedies can include avoiding foods or beverages that irritate the ulcers, as well as taking medications (e.g., mouth rinses, topical anesthetics and over-the-counter pain medications) to reduce the pain and discomfort of canker sores. Home remedies are also available.

Guaranteed prevention of canker sores is not possible, although there are certain actions people can take to reduce the likelihood of developing these sores. Such actions include maintaining good health, reducing stress, avoiding possible triggers (e.g., hot, spicy, acidic, hard or salty foods) and practicing good dental hygiene.

About canker sores

Canker sores are shallow, open sores (ulcers) that develop inside the mouth. These painful ulcers can appear on the inner cheeks, inside lip, base of the gums, or on or under the tongue. The presence of canker sores in these areas can make it uncomfortable to eat, drink and/or talk.

Although canker sores are irritating and painful, they are generally harmless. Most often, they resolve without treatment within approximately two weeks. Canker sores are not contagious, although they often recur in people who have previously had them.

Canker sores are a common type of mouth sore. Approximately one in every five people experience recurring canker sores, according to the American Academy of Family Physicians. They may occur at any time in a person’s life, although they appear most often during adolescence and early adulthood. Smoking tobacco does not appear to increase the risk of developing canker sores.

The exact process is unknown, but canker sores are believed to form due to an irregular response by the body’s immune system. Once triggered, white blood cells attack and damage tissue lining the mouth, creating inflamed cavities known as canker sores.

Canker sores may occasionally indicate an underlying medical condition such as connective tissue diseases (e.g., lupus), blood diseases (e.g., anemia), drug reactions (e.g., allergies, side effects), inflammatory skin disorders or cancer.

Canker sores can sometimes lead to infection. Bacteria may build up inside an ulcer, causing a canker sore to become infected. Reasons for this include poor dental hygiene and inserting dirty fingers into the mouth. A fever may indicate that a canker sore has become infected. In addition, difficulty swallowing due to the pain of canker sores may lead to inadequate fluid intake and dehydration. 

Canker sores are often confused with cold sores, a different type of mouth sore. While cold sores are contagious, canker sores are not. The two also differ in a number of other significant ways:

  • Location. Canker sores appear inside the mouth, whereas cold sores typically appear outside the mouth (e.g., in the lip area).

  • Appearance. Canker sores are pale, open sores that look like craters in the skin and feature a red border. Cold sores are blisters that burst and crust over.

  • Cause. Cold sores are caused by herpes simplex viruses and canker sores are not.

Types and differences of canker sores

Canker sores can be divided into three different types, based primarily on their size. Types of canker sores include:

  • Minor aphthous ulcers. Less than 1 centimeter (cm) in diameter. These are small ulcers with a flat red border that may have a white, yellow or gray center. They typically leave no scars and usually heal within two weeks. These are the most common type of canker sores.

  • Major aphthous ulcers, also known as Sutton’s disease. Greater than 1 cm (0.5 inch) in diameter. These are very large ulcers with a raised red border. They can take up to a month to heal and may leave scars. These canker sores are uncommon.

  • Herpetiform ulcers. Less than 3 millimeters (mm) in diameter. These are clusters of tiny ulcers that are otherwise similar to minor aphthous ulcers. These ulcers may join to form patches in the mouth, and usually heal in about one week. This type of canker sore is also rare.

Risk factors and causes of canker sores

The exact cause of canker sores is unclear. The sores appear to occur when the body’s immune system attacks the tissue lining the mouth, creating ulcers. Why this happens is unknown.

Although the cause is unknown, certain factors appear to make a person more susceptible to canker sores. These include:

  • Family history. Canker sores appear to run in families.

  • Gender. Canker sores are more common in women than in men.

  • Age. Although they may appear anytime during a person’s life, canker sores most often appear during adolescence and early adulthood.

Likewise, certain factors appear to trigger an outbreak of canker sores. These include:

  • Tissue injury or trauma. The sensitive lining of the mouth may be injured in a variety of ways, all of which may trigger canker sores. This includes damage caused by:
    • Bites on the inner cheek, tongue or lip
    • Sharp or broken teeth
    • Braces
    • Ill-fitting dentures
    • Chewing tobacco
    • Burns from hot foods or liquids

  • Immune system weakness or deficiency. For example, canker sores tend to appear while the body is busy fighting bacterial or viral infections (e.g., during a cold or the flu) or in people with the human immunodeficiency virus (HIV), acquired immunodeficiency syndrome (AIDS) or cancer.

  • Stress, fatigue. The appearance of canker sores seems to be related to periods of stress or feeling fatigued or run down.

  • Vitamin deficiency. A lack of certain vitamins (e.g., vitamin C, vitamin B12, iron, folic acid, zinc) may be related to the onset of canker sores.

  • Allergies. Allergic reactions may trigger canker sores.

  • Medication side effects. Certain medications (e.g., chemotherapy drugs, NSAIDs) appear to trigger the onset of canker sores.

  • Hormones. Canker sores tend to appear just before a woman’s menstrual period, and tend to disappear during pregnancy.

  • Intestinal problems. Flare-ups of certain digestive disorders (e.g., Crohn’s disease, ulcerative colitis, celiac disease) may trigger canker sores.

Signs and symptoms of canker sores

Canker sores appear on the inside of the mouth. They can be found on the inner cheeks, inside the lip, at the base of the gums, and on or under the tongue. There may be a single sore or they can appear as a cluster of many sores. They are almost always painful. Mouth ulcers that are not painful may be a sign of oral cancer.

Canker sores have a distinct appearance. These open sores (ulcers) look like small craters in the mouth with the following qualities:

  • Round (circular in shape – not oblong)
  • Shallow (ulcer affects only the uppermost layer of the lining of the mouth)
  • Small (usually less than 1 centimeter in diameter)
  • Center of sore is white, gray or pale yellow in color
  • Edges of the sore are red

Less common symptoms that can accompany canker sores include fever, swollen lymph nodes and feeling sluggish or ill.

Canker sores are typically preceded by a burning or tingling in the area where a sore will develop. Within six to 24 hours, a red spot or bump appears in the area. Within a few days, it bursts and leaves an open, shallow sore that enlarges. The pain of a canker sore usually lessens in the first few days, although the sores can last up to two weeks before spontaneously healing.

Certain symptoms or changes in symptoms may indicate an underlying medical condition that requires treatment. When symptoms are severe, prescription medication may be necessary for relief or other treatment options may need to be explored. Patients with canker sores should seek immediate medical attention – from their dentist or other health care provider – when the following occurs:

  • Sores are extremely painful
  • It becomes difficult to talk, chew or swallow
  • Sores have not healed after two weeks
  • Sores are very large
  • Sores recur frequently
  • Sores appear to be spreading
  • Fever is present (this may also indicate that a canker sore has become infected)

Diagnosis methods for canker sores

A dentist or other health care provider can usually identify canker sores through visual examination of the affected area (e.g., inner cheeks, lip, at the base of the gums, on/under the tongue). 

A medical history may also be collected, during which a patient may be asked the following questions:

  • Have you had canker sores in the past?
  • Do you have a family history of canker sores?
  • Has any area of your mouth been injured recently?
  • What medications are you currently taking?
  • What types of foods or beverages do you consume?
  • Has your diet changed recently?
  • How often do you brush and floss?
  • Are you experiencing any additional symptoms (e.g., fever, fatigue)?

Additional tests, such as blood tests or biopsy are sometimes performed, although they are not usually necessary in otherwise healthy patients. These tests may help identify or rule out other potential conditions or diseases that can cause canker sores (e.g., nutritional deficiencies, immune system problems, allergies).

Treatment options for canker sores

In most cases, canker sores heal without treatment within approximately two weeks. There is no cure for canker sores. Therefore, treatment is usually focused on alleviating a patient’s symptoms.

Avoiding foods or beverages that can irritate the inside of the mouth may reduce the pain and discomfort that accompany canker sores. This includes anything acidic (e.g., soft drinks, tomatoes, strawberries, citrus fruits or juice), abrasive (e.g., potato chips, nuts), hot or spicy. Some dentists recommend aloe vera to ease the pain of canker sores.

While a canker sore is healing, patients should continue to practice good dental hygiene. This prevents the buildup of food particles in the mouth, which may worsen the condition. Ice can be applied to canker sores to numb the area and temporarily relieve pain. A straw may be used when drinking to prevent beverages from coming into contact with and further irritating existing sores.  

Medications may also be used to help alleviate a patient’s symptoms. Topical solutions (e.g., ointments, mouth rinses) are usually tried before use of systemic drugs, which may carry more serious side effects. These medications are available by prescription or over-the-counter and may include:

  • Antimicrobial mouth rinses. Medicated mouthwash that fights infection to keep the area clean.

  • Medications that coat the mouth. Products that contain glycerin (a natural moisturizer) can reduce pain by protecting existing sores against further irritation.

  • Carbamide peroxide. A combination of hydrogen peroxide (a disinfectant) and glycerin. This over-the-counter medication both cleans and protects the affected area.

  • Topical anesthetics. These numb the area to help reduce pain.

  • Over-the-counter pain medication (e.g., acetaminophen, ibuprofen, aspirin). These can reduce pain and may also reduce inflammation. Aspirin should never be given to children or teens due to an increased risk of Reye syndrome, a potentially life-threatening illness.

Mouth rinses usually require a “swish and spit” application. A small amount of the liquid is placed in the mouth and swished around with movement of the cheeks and tongue. This is done for about a minute before the liquid is spit out. Most of these types of medications should not be swallowed (unless advised by a physician) and may be used every four to six hours.

In order to apply topical medications to canker sores, patients should first blot the affected area dry with a tissue. A cotton swab can be used to apply a small portion of the medication directly on the sore. Patients should not eat or drink for at least 30 minutes after application to avoid washing away the medication. This type of medication is usually applied four times a day (or after meals, upon awakening and before going to sleep).

In severe cases, antibiotics or steroid medications may be prescribed. Canker sores may also be burned off (e.g., with laser therapy) by a dentist or other physician, in which case they should disappear completely within a few days.

Home remedies may also provide relief from canker sore symptoms. These treatments focus on the use of bacteria-inhibiting antiseptics and pain-relieving antacids that can be purchased without a prescription. For best results, the use of an antiseptic should be followed by the use of an antacid. These remedies include:

  • Hydrogen peroxide solution. A solution of one part hydrogen peroxide with one part water can be dabbed directly onto canker sores or used as a mouth rinse (although it should not be swallowed). Peroxide fights bacteria that can accumulate in an ulcer.

  • Antacids. There are a variety of ways to use over-the-counter antacids to soothe canker sore pain. These include:
    • Magnesium hydroxide (e.g., Milk of Magnesia). Commonly used as an antacid or laxative. Coating canker sores with a small amount of magnesium hydroxide several times a day may bring some relief.

    • Other liquid antacids. These may be used directly on canker sores or as a mouth rinse.

    • Homemade antacid. Baking soda and water can be combined into a paste for direct application to canker sores, or they can be made into a mouth rinse (1 teaspoon of baking soda and 1 cup of warm water).

  • Magnesium hydroxide solution. Combining equal parts magnesium hydroxide and liquid diphenhydramine hydrochloride (e.g., Benadryl), an over-the-counter allergy medication, creates a mouth rinse that may be soothing for people with canker sores. The allergy medication acts as a topical anesthetic and contains antihistamines, which can reduce pain and inflammation.

  • Tea bags. Although this may be uncomfortable, placing wet tea bags (which contain tannin, a natural astringent) on the sores may help relieve pain.

Some people have reported faster healing time of their canker sores by using the following methods, although there is little evidence to support these claims:

  • Zinc lozenges
  • Vitamin C
  • B-complex vitamins
  • Sage-and-chamomile mouthwash
  • Lysine supplements

If an underlying condition (e.g., allergies, anemia) is responsible for the canker sores, it is important to identify and treat that condition.

Prevention methods for canker sores

There is no guaranteed way to prevent canker sores. They often recur in people who have previously had them.

However, there are methods people can take to help reduce the likelihood of an outbreak. Foremost among these is maintaining good health. This includes eating a healthy diet, getting plenty of rest and drinking lots of fluids. It can also involve stress reduction techniques such as removing oneself from stressful environments and/or engaging in activities that help a person better manage the stress in their life. These techniques may include regular exercise, meditation, deep breathing, guided imagery and yoga.

In addition, the following tips may help prevent canker sore outbreaks:

To prevent canker sores that may develop as a result of infection:

  • Brush and floss regularly
  • Eat live-culture yogurt (it helps to balance bacteria levels in the body)

To prevent canker sores triggered by tissue irritation:

  • Avoid products that can irritate the mouth (e.g., hot, spicy, acidic, hard or salty foods)
  • Use a soft-bristled toothbrush or foam swabs
  • Try switching to a toothpaste without sodium lauryl sulfate (SLS) or other irritating ingredients

To prevent canker sores triggered by nutritional deficiencies, take appropriate vitamin and mineral supplements (e.g., vitamin C, vitamin B complex, folic acid, iron, zinc).

Questions for your doctor about canker sores

Preparing questions in advance can help patients have more meaningful discussions with their dentists regarding their conditions. Patients may wish to ask their doctor the following questions related to canker sores:

  1. How can I tell if I have a canker sore or a cold sore?
  2. Does my canker sore need treatment?
  3. What type of over-the-counter remedies do you recommend to help alleviate the pain associated with my canker sores? Are there any I should avoid?
  4. What types of foods or drinks should I avoid while I have canker sores?
  5. Are there home remedies I can use to minimize my pain?
  6. How long can I expect my canker sores to last?
  7. What changes in my symptoms should I report to you?
  8. Could some underlying condition (e.g., vitamin deficiency, allergies, immune system problems) be causing my canker sores?
  9. Can stress be related to my canker sores? Will reducing my stress level prevent future outbreaks?
  10. Do you recommend I switch my toothpaste if I am prone to canker sores? What type of toothpaste do you recommend?
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