Canker sores, also known as aphthous ulcers, are small, pale, round, painful lesions that may occur inside the mouth. They are not contagious.

People may get canker sores on the:

  • cheeks
  • lips
  • tongue
  • fleshy part of the back of the roof of the mouth

According to the American Academy of Oral Medicine, more than half of the population of the United States experiences a canker sore at some time.

Scientists have not yet determined what causes canker sores. Although many treatments are available, there is no cure.

In this article, we examine whether canker sores are contagious and how they occur. We also look at how to treat and prevent them.

a senior couple kissing where there is not risk of spreading canker sore infections as they are not contagiousShare on Pinterest
Canker sores are not contagious and will not spread from person to person.

Researchers do not know what causes canker sores, but they do not spread from person to person, even as a result of close contact. Canker sores are, therefore, not contagious.

Canker sores do tend to come back, sometimes several times a year. While this may make it seem as though they are contagious, they are not.

People often confuse canker sores with cold sores, but, aside from their small, painful, and bumpy appearance, they are quite different.

People usually notice cold sores outside the mouth and around the lips, but canker sores are always inside the mouth. Cold sores appear transparent at first but become cloudy as they fill with fluid, which can be yellow and resemble pus. Canker sores are gray-white with a red outer ring, and inflammation surrounds them.

The herpes simplex virus (HSV-1) causes cold sores. These sores are quite contagious, and any contact with the blister can cause the virus to spread to another person or another part of the body.

Researchers have not identified a specific reason why canker sores develop. However, some possibilities may include:

  • an allergic reaction
  • autoimmune disorders
  • a reaction to acidic foods
  • an injury to the mouth tissue
  • gluten intolerance
  • smoking
  • stress
  • nutritional deficiency in vitamin B12, folate, or iron
  • menstrual cycles

Genetic factors may also play a role in the development of canker sores because they tend to run in families.

Some researchers suggest that canker sores could signal gastrointestinal problems, such as:

  • celiac disease or gluten intolerance
  • Crohn’s disease
  • ulcerative colitis

It is most common for people to get canker sores for the first time between 10–19 years of age. Recurrent episodes tend to become less frequent as people get older.

The life cycle of a canker sore eruption — from the first signs of a problem to healing — is about 10–15 days. They sometimes come back to the same or nearby locations several times a year and can form in groups of two to four.

A few hours before the canker sore forms, some people may feel burning or tingling in the affected area. The area will then become inflamed, swollen, and sore. In the next few days, the swelling becomes more defined and starts to take on the familiar sore shape with raised edges.

There are several types of canker sores:

  • Minor canker sores are typically just a few millimeters wide. These are the most common form, affecting 85% of people who get canker sores.
  • About 10% of people with canker sores develop major canker sores, which can be as big as 1–3 centimeters.
  • Only 5% of people with canker sores get herpetiform canker sores, which are clusters of many tiny lesions.

Canker sores are usually at their most painful for 3–5 days at the start of their cycle. Major canker sores and herpetiform canker sores may take longer to heal and could leave scars.

As experts have not found a definitive cause for canker sores, there are no specific guidelines for preventing them.

Individuals may wish to track their outbreaks of canker sores and see whether these correlate with specific factors, such as those relating to:

  • diet
  • smoking habits
  • stress levels
  • menstrual cycle

A survey in India examining the relationships between lifestyle, diet, and canker sore formation found that:

  • 100% of people identified a link with high stress levels
  • 100% of people had eaten spicy food
  • 80% linked the consumption of gluten-rich food to the appearance of the sores
  • 21% were menstruating when the sores developed

There is no cure for canker sores. However, people can use different techniques to make themselves more comfortable and lower the risk of lesions becoming infected. These techniques include:

  • avoiding spicy foods or other foods that can irritate the sores, such as crusty bread, salty or acidic dishes, and fizzy beverages
  • using a softer toothbrush and being gentle when brushing the teeth and flossing
  • using a toothpaste that does not contain sodium lauryl sulfate (SLS)
  • using over-the-counter (OTC) products, such as benzocaine, to numb sore areas
  • rinsing or gargling with antiseptic solutions, such as a mixture of half hydrogen peroxide and half water or 4 ounces of water mixed with 1 tablespoon (tbsp) salt and 1 tbsp baking soda

Most of the time, canker sores get better without intervention. However, if the pain is severe, and the recurrences are frequent, people may wish to seek medical care.

Doctors may prescribe:

  • chlorhexidine (Paroex) mouth rinse
  • corticosteroids as a rinse or ointment
  • corticosteroid tablets
  • laser treatment

One study found ozonated oil to be effective in providing relief from recurring canker sores.

Canker sores are among the most common health issues affecting the mucous membranes of the mouth, but scientists do not know what causes them or how to cure them.

Most of the time, canker sores do not require treatment from a healthcare professional. Many different OTC options are available to reduce discomfort and prevent complications, such as infections. Canker sores are not contagious.