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What Is a Canker Sore? The Ultimate Guide

A woman in a red shirt closes her eyes and holds her lip in pain because of a canker sore.Most people deal with a canker sore at some point in their lives. It might cause you to favor one side of your mouth when you chew or make you hold your mouth differently when you speak.

But what exactly is a canker sore? What causes canker sores? Are canker sores herpes? And, for readers currently dealing with these mouth ulcers, how long do canker sores last?

What Is a Canker Sore?

A canker sore (also called an aphthous ulcer) is a shallow ulcer that develops inside the lips and mouth. These white, gray or yellow lesions usually measure between one and four millimeters in diameter and have a bright red or pink border.

Though they can be extremely painful, canker sores are not dangerous. They also aren't caused by a virus, so they can't spread from person to person through kissing or other contact.

Canker sores are very common, and many people deal with them several times per year. People sometimes experience a stinging or burning sensation in the affected area immediately before a canker sore emerges. The ulcers usually hurt most in the first few days after they form.

The 3 Stages of a Canker Sore

Canker sores progress through three stages before they disappear, and their appearance differs in each stage.

1. Prodromal Stage

For the first one to three days, you often can't even see a sore in your mouth. However, you may experience burning or tingling in the spot where the canker sore will eventually form. Toward the end of this period, a small ulcer develops in your mouth.

2. Ulcer Stage

During days three through six, the canker sore lesion grows to its full size. This process generally takes around 96 hours. The canker sore becomes very painful during this time.

3. Healing Stage

After about day six, healing begins. This period generally sees the canker sore decreasing in both size and pain. Most canker sores are completely healed within 14 days.

For more detailed information on how to recognize a canker sore, check out our canker sore identification guide.

How Long Do Canker Sores Last?

Although healing time varies widely from person to person, normal canker sores usually heal within 14 days from that first painful burning sensation. Some canker sores, however, can last much longer.

Complex Canker Sores

Complex canker sores can last for up to six weeks at a time. These complex lesions, which can exceed five millimeters in size, usually form in response to an underlying health issue. Some of the more common triggers for complex canker sores include autoimmune disorders, food allergies and vitamin deficiencies.

These more serious canker sores can also be accompanied by flu-like symptoms such as swollen lymph nodes, fever and low energy.

Are Canker Sores Herpes?

Canker sores are often confused with cold sores, which are caused by the herpes simplex virus. But canker sores are not caused by a herpes virus, or by any virus.

Location can help you tell the two apart. Cold sores usually appear on the outside of the lips and mouth, and canker sores always appear inside the mouth.

Even herpetiform canker sores, despite their name, are not caused by a herpes virus. Herpetiform sores get their name because of their resemblance to a cluster of herpes blisters.

What Causes Canker Sores?

Canker sores don't have one single, known cause, which makes them somewhat difficult to combat. However, we do know about many of the potential triggers and risk factors for canker sores, which can include:

  • Low levels of vitamin B12, iron and zinc.
  • Autoimmune conditions like lupus.
  • Use of nonsteroidal anti-inflammatory drugs (NSAIDs).
  • Family history of canker sores.
  • Injury to the sensitive lining of the mouth.
  • Hormonal changes, such as pregnancy.

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Dietary Triggers

Foods can cause canker sores in a variety of ways. Any foods that irritate or injure the oral tissues can potentially lead to a canker sore. This can include:

  • Acidic fruits and vegetables, including juices.
  • Spicy foods.
  • Salty foods.
  • Crunchy or sharp foods.
  • Coffee.
  • Alcohol.
  • Any food a person is allergic to.


Although researchers haven't conclusively identified stress as a cause of canker sores, they have found a high correlation between recurrent canker sores and anxiety, depression and psychological stress.

How To Get Rid of Canker Sores

Fortunately, many options exist for treating painful canker sores.

Home Remedies

Anecdotal evidence suggests the benefits of many home remedies for canker sores. While medical proof may not exist (yet!) for many of these remedies, people swear by the use of several ordinary household products for the treatment of canker sores. Here are just a few:

  • Coconut oil.
  • Apple cider vinegar.
  • Honey.

Red Light Therapy

Red light therapy can also help to reduce pain and speed healing of canker sores. In fact, one study showed that 75% of patients treated with red light reported reduced pain after one session and experienced a shortened healing time of just four days.

While treatment with red light used to be available only in a clinical setting, many products now exist to harness this technology for home use. The Luminance RED is an FDA-registered device designed to use this kind of red light therapy for canker sores at home.

Lysine Supplementation

Lysine is an essential amino acid that functions as a building block for proteins. Your body can't make lysine on its own, so you have to consume lysine in your diet.

Lysine has many important functions in the body, including:

  • Immune system support.
  • Nutrient absorption.
  • Collagen production.

All of these functions could help improve existing canker sores and decrease the likelihood of developing new ulcers.

Over-the-Counter Medications

Many over-the-counter canker sore treatments are available in the form of patches, topical creams or dental products at your local drugstore.

Topical ointments, liquids and creams use a variety of ingredients to soothe canker sore pain and heal and tighten the skin affected by the ulcer.

Patches can be applied directly to the canker sore to protect it from further irritation. There are even specially formulated toothpastes to help combat bacteria that can aggravate canker sores.

When To See a Doctor About Canker Sores

If you've made lifestyle changes and tried treatments at home but still suffer from frequent and recurrent canker sores, it may be time to call a medical professional. Mouth ulcers that cause severe pain, sores that spread to other areas of the mouth, or unusually large sores that are not healing are also signs that you need to contact your doctor.

Recurrent canker sores, especially severe ones, can be a symptom of a serious underlying health condition such as Crohn's disease, AIDS or celiac disease. Your health care provider will be able to test you for these conditions as well as order blood work to check for deficiencies in certain vitamins and minerals.

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