Canker Sore (Aphthous Ulcer)

Medically Reviewed by Robert Brennan, DDS on September 05, 2023
4 min read

A canker sore is a small, shallow open wound (or ulcer) in your mouth that can make eating and talking uncomfortable. They’re also known as aphthous ulcers. There are a few types of canker sores:

  • Minor canker sores. These may show up three or four times a year. They typically happen in people ages 10 to 20. They’re less than 1 centimeter across and heal in about a week with no scarring.
  • Major canker sores. These are less common. The ulcers are bigger and can last more than 2 weeks. They often heal with scarring.
  • Herpetiform canker sores. These are rare and show up as clusters of tiny ulcers. They usually heal in about a week.

Doctors don’t know what exactly causes most canker sores. 

Things that might cause minor sores include:

  • Stress 
  • Tissue injury, like from a sharp tooth or a dental appliance
  • Certain foods, including citrus or acidic fruits and vegetables (such as lemons, oranges, pineapples, apples, figs, tomatoes, and strawberries)
  • Nonsteroidal anti-inflammatory drugs like ibuprofen
  • An allergy to something in your food or in your toothpaste or mouthwash
  • Helicobacter pylori, the bacteria that can cause peptic ulcers

Things that might cause complex canker sores include:

  • An underlying health condition like a weakened immune system, HIV/AIDS, lupus, or Behcet’s disease
  • Nutritional problems like too little vitamin B12, zinc, folic acid, or iron
  • Gastrointestinal diseases like celiac or Crohn’s

About 1 in 5 people gets canker sores regularly. They’re more common in women, possibly because of hormonal differences. They may also run in families.

Canker and cold sores aren’t the same. 

Cold sores, also called fever blisters or herpes simplex type 1, are groups of painful, fluid-filled blisters. Unlike canker sores, a virus causes cold sores, and they’re highly contagious. Also, cold sores typically appear outside your mouth -- usually under your nose, around your lips, or under your chin -- but canker sores show up inside your mouth.

You may have a canker sore if you have:

  • A tingling or burning sensation, often 6 to 24 hours before a canker sore
  • Small sores in your mouth that are round or oval, white, gray, or pale yellow with a red edge or border
  • A painful sore or sores that appear alone or in clusters inside your mouth: on your tongue, at the base of the gums, on your soft palate (the back portion of the roof of your mouth), or inside your cheeks

Severe canker sore attacks may also cause:

  • Fever
  • Fatigue
  • Swollen lymph nodes


Canker sores usually aren’t serious. Call your dentist if you have:

They can usually make a diagnosis based on a physical exam and your medical history. They might want to test your blood to see whether a vitamin deficiency or another health condition is causing the sores.

Pain from a canker sore tends to get better in a few days, and the sores usually heal without treatment in about a week or two. Treatment for large, long-lasting, or unusually painful sores might include:

  • Mouthwashes. Your doctor can prescribe a rinse that has a steroid or a painkiller.
  • Topical medications. Your doctor may prescribe a topical medication containing a steroid for the inflammation and a topical analgesic like lidocaine to relieve pain. Triamcinolone acetonide or clobetasol are often used to reduce pain and inflammation from canker sores.
  • Oral medications. The ulcer drug sucralfate (Carafate) and the gout drug colchicine (Mitigare) can also treat canker sores. Or your doctor might give you steroid pills.
  • Nutritional supplements. You might need these if a nutrient deficiency is causing your canker sores.   
  • Cautery. Dental lasers can help you feel better right away. Your doctor can also cauterize sores with chemicals like debacterol or silver nitrate.

Home treatments to speed healing and help you feel better include:

  • Topical products. Medicated gels, creams, pastes, and liquids go on the sore.
  • Mouth rinses. Mix salt or baking soda in warm water, and swish it around your mouth.
  • Milk of magnesia. Put a bit on a cotton swab, and dab it on the sore.

There’s no cure for canker sores, and they often come back. But you might get them less often if you:

  • Avoid foods that irritate your mouth, including citrus fruits, acidic vegetables, and spicy foods.
  • Don’t chew gum.
  • Brush your teeth with a soft-bristled brush after meals, and floss daily, which will keep your mouth free of foods that might trigger a sore.