Fact Checked

What Is The White Stuff In A Canker Sore?

Written by Fernanda Elizalde

December 08, 2022

Medically Reviewed

By Dr. James Younan, DDS

It’s never fun when you’re in the middle of a great meal and suddenly feel a tender area in your mouth that wasn’t there the day before. If you look in the mirror and find that the source of your pain is a small white spot inside your mouth, you’ve likely got a canker sore.

Most people have had a canker sore at some point in their lives. However, despite their prevalence, these tiny mouth lesions are largely misunderstood and may be mistaken for other conditions.

If you’re dealing with a canker sore at the moment, it’s completely normal to worry about how quickly it’s healing and how to treat it at home. In addition, you’ve also probably noticed a distinct white substance inside the wound.

So, what is the white stuff in a canker sore, and what’s the best way to treat them? In this article, we’ll discuss what a canker sore is, what that white stuff is inside them, and the best ways to treat them.

tongue cleanser

What is a canker sore?

Canker sores, also known as aphthous ulcers, are one of the most common causes of white spots inside the mouth, affecting about 20% of the U.S. population at least once in their lifetime.

These small, painful sores typically start as red bumps but grow into white or yellowish ulcers with a red border. They are usually very small, less than a millimeter, but may grow to ½ or even a full inch in diameter.

Because they only occur inside the mouth, most people will develop canker sores on the pink tissue inside their cheeks or lips. They’re also commonly found on the gums, tongue, or roof of the mouth.

Before a canker sore appears, you’ll likely notice a slight burning or tingling sensation along with other symptoms, such as swollen lymph nodes or fever. Once visible, canker sores are often accompanied by stinging pain, which is exacerbated by eating acidic or spicy foods. However, there are several types of canker sores, varying in size, shape, and pain level.

The three most prevalent types of canker sores are minor, major, and herpetiform sores.

Minor canker sores

If you have a canker sore, it’s most likely of the minor variety, which affects more than 80% of sore sufferers. Minor canker sores usually:

  • Cause a tingling or burning sensation when eating, drinking, or talking
  • Are small, less than 5 mm in diameter
  • Are oval-shaped, with a red edge
  • Heal in about 7 to 14 days
  • Heal without scarring

Major canker sores

  • Major canker sores are less common than minor ones, and:
  • Are usually round with defined borders but may have irregular edges
  • Can be extremely painful, making it difficult to speak or eat
  • Are larger and deeper, over 1 cm in diameter
  • May take up several weeks or months to heal
  • Often require medical treatment
  • Can leave extensive scarring

Herpetiform canker sores

Herpetiform canker sores are the least common and usually develop later in life. Additionally, despite their similar name, herpetiform canker sores (like all other canker sores) are not caused by the herpes virus. These aphthous ulcers:

  • Often develop in clusters of 10 to 100 sores, which may join to form one large ulcer
  • Are pinpoint size, less than a few millimeters in diameter
  • Are typically extremely painful
  • May require medical treatment
  • Usually heal without scarring
  • Heal in about a month
  • Have irregular edges

Snow Whitening Mouthwash
What is the white stuff in a canker sore?

A layer of “white stuff” is normal in the later stages of a canker sore. Still, many people find the white substance particularly uncomfortable and embarrassing. So, what exactly is this white stuff, and why does it form?

Although it’s not very pleasant to look at, this substance is a critical component of the healing process called fibrin membrane. Fibrin is a protective protein that forms as a natural response to injuries in the body.

For example, when tissue in the mouth is injured, such as when a canker sore forms, the body produces white blood cells to fight the infection and help seal the wound. These white blood cells produce the sticky material known as fibrin, which accumulates in canker sores and gives them their white or yellowish color.

It may not look pretty, but the role of fibrin is to act as a type of scar tissue, helping to protect the sore while it’s healing.

Canker sores vs. cold sores – are they the same?

Canker sores are frequently mistaken for cold sores and vice versa. However, although they are common across all ages and have similar symptoms, they are separate conditions with entirely different causes.

Cold sores, also known as fever blisters, are painful fluid-filled sores that form on the outside of the mouth, usually on the borders of the lips. They are caused by the herpes simplex virus, usually HSV-1. Unlike cold sores, canker sores are simply a form of inflammation and are not contagious.

It’s estimated that nearly 90% of all U.S. adults have been infected with the herpes virus, but many people never show symptoms.

This is because the virus generally resides in a dormant state within the nerve cells, waiting for the body to be immunocompromised, like when you’re stressed out or suffering from a cold. Once it reactivates, the virus travels to the skin, causing the infamous recurring blisters.

What can trigger a canker sore?

A combination of factors may trigger the formation of a canker sore, including:

  • Oral injuries (irritation after dental work, sports injuries, accidental cheek biting, etc.)
  • Certain conditions and diseases, such as HIV/AIDS, celiac disease, and Crohn’s disease
  • Dental appliances, such as braces or ill-fitting dentures
  • Excessively brushing your teeth
  • Nutritional deficiencies
  • Acidic and spicy foods
  • Hormonal changes
  • Stress and fatigue
  • Poor oral hygiene
  • Food allergies

How to get rid of canker sores fast

There is no cure or treatment for canker sores that can make them permanently disappear. Most treatments focus on minimizing the pain and speeding up the healing process. Most canker sores can often be treated with one of the following home remedies:

  • Apply a mixture of half water and half hydrogen peroxide to the sore using a cotton swab.
  • Rinse your mouth with a mixture of half Milk of Magnesia and half Benadryl liquid allergy medicine.
  • Take anti-inflammatory medicine, such as ibuprofen (Advil, Motrin) or naproxen (Aleve), to reduce pain.
  • Use an over-the-counter numbing mouth rinse or topical drops for short-term pain relief.
  • Rinse your mouth with salt water or a mild, alcohol-free, over-the-counter mouthwash.
  • Apply ice to the sore to reduce pain and swelling.

The previous home remedies are helpful for minor canker sores. However, professional medical treatment may be necessary for particularly painful or severe canker sores.

A doctor can prescribe more effective treatments, including Chlorhexidine mouthwash, topical anesthetics, such as benzocaine, and oral medications or steroids to reduce pain and inflammation.


Canker sores are a common form of mouth ulcers that are typically small, round, and have a white or yellowish center with a red border. The sores are often accompanied by a stinging or burning sensation, which may be worsened by spicy or acidic foods.

The exact cause behind canker sores is unknown. However, many factors may play a role, including stress, fatigue, oral injuries, or hormonal changes.

Additionally, although most canker sores heal on their own, canker sores can often be treated with home remedies like a saltwater rinse or over-the-counter topical anesthetic.


Is it good if a canker sore is white?

The white stuff in a canker sore is a protein known as fibrin, which plays a vital role in healing. Although it may be unsightly, fibrin is a good sign that your body is healing the canker sore on its own.

Do canker sores turn white when healing?

It’s normal for a canker sore to turn white when healing. This is typically the final stage of the healing process, occurring once the healing tissue starts to close over the mouth ulcer and pain has begun to subside.

Do canker sores have pus in them?

Unlike dental abscesses and boils, canker sores do not normally have pus. However, in some cases, an infected canker may ooze pus. If this happens, you should contact a doctor immediately for professional medical treatment.

Is it normal for a canker sore to pop?

Doctors advise against popping canker sores. This is because popping a canker sore can be very painful and will likely only further irritate the painful open wound.

snow teeth whitening kit