Symptoms of canker sores

Canker sores are small lesions in your mouth. Canker sores usually last between 1 and 2 weeks and you get rid of them fast by using over-the-counter medications to treat them.
Canker sores are small lesions in your mouth. Canker sores usually last between 1 and 2 weeks and you get rid of them fast by using over-the-counter medications to treat them.

Canker sores are small lesions in your mouth. They are usually small, white, and can be painful. In rare cases, they can be large. They appear inside your mouth or on your gums, not on your lips. Canker sores usually last between 1 and 2 weeks. Large sores can last up to 6 weeks.

Before you get a canker sore, you may feel a tickling, burning, or tingling sensation in the spot where the sore later shows up. Once the sore develops, it will be round with a white or yellowish center. 

Most canker sores are less than 1 millimeter in diameter. However, they can be as big as 1 inch in diameter. When larger, they may have a more irregular shape and can last longer. Larger canker sores may also leave scars once healed. They can occur alone or in groups.

Canker sores are usually painful and may make it harder to eat or talk, especially if they are large. In more severe cases, you may also experience a fever or general ill feeling.

Canker sores are not contagious.

Causes of canker sores

Experts are not sure about the exact causes of canker sores, but they believe many factors may cause them to develop. These include:

  • Mouth injuries 
  • Allergic reaction to food or bacteria 
  • Oral health products that contain sodium lauryl sulfate
  • Low B-12 levels 
  • Low folate levels 
  • Low iron levels 
  • An H. Pylori bacterial infection 
  • Hormonal changes 
  • Too much stress 
  • Eating too many citrus fruits 
  • Eating too many acidic foods like tomatoes 
  • Using non-steroidal anti-inflammatory medications 
  • Conditions that affect the immune system like lupus and celiac disease

How to treat canker sores fast

A few over-the-counter products may speed healing and help with pain or discomfort. These products work best if you use them as soon as you feel a canker sore coming on.

  • Hydrogen peroxide mouth rinse
  • Fluicinocide
  • Benzocaine

When to see your doctor about canker sores

Most canker sores go away on their own without treatment. However, you should see your doctor or dentist if your canker sores:

  • Are large
  • Recur frequently
  • Often develop before your old ones are fully healed
  • Last longer than 2 weeks
  • Appear on your lips instead of inside your mouth
  • Are extremely painful
  • Make it hard to eat or drink
  • Come with an associated fever

Your doctor may prescribe dexamethasone, a mouthwash that can reduce inflammation and pain

In persistent cases, your doctor may prescribe steroids. Other prescription medications that can help include sucralfate, which is usually used to treat ulcers, or colchicine, often used to treat gout.

In rare cases, your doctor or dentist may need to cauterize your canker sore. In this process, the tissue is burned or destroyed using heat or chemicals to get rid of the canker sore.

If you have persistent canker sores, your doctor may recommend that you take B-12, folate, or iron supplements. They may also investigate whether you have an underlying health issue that causes your sores to come back.

How to tell the difference between canker sores and cold sores

Some people confuse canker sores with cold sores, even though they are quite different.

Cold sores are caused by the herpes virus. They are quite contagious when they form gooey blisters and open sores, and less contagious otherwise. Cold sores often occur in groups and appear as small, fluid-filled bumps. They may tingle or burn when they first appear.

When left untreated, the bumps burst, releasing fluid that then crusts over. They often appear when you are already sick or stressed or if you have a weakened immune system. They most often occur outside of the mouth but can show up inside as well.

Cold sores look different from canker sores and have different causes. Cold sores are contagious. Canker sores are not. If you're not sure which you have, your doctor can perform a visual examination to make a definitive diagnosis.

How to prevent canker sores

If you get canker sores, there are steps you can take to prevent them from coming back.

  • Avoid eating foods that irritate your mouth
  • Eat a balanced diet
  • Brush your teeth and floss regularly
  • Avoid toothpaste and mouthwashes that contain sodium lauryl sulfate
  • Use orthodontic wax to protect your mouth if you have braces
  • Try to reduce your stress
  • Manage any underlying conditions that cause canker sores


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Medically Reviewed on 12/28/2021

Cleveland Clinic: "Canker Sores." "Canker sores (mouth ulcers): What can you do if you have a canker sore?"

Mayo Clinic: "Canker sore," "Cold sore."

Penn Medicine: "Cold Sores vs. Canker Sores: What Are They and How Do I Get Rid of 'Em?"