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What To Do if Your Canker Sore Keeps Getting Bigger

A woman looks in a mirror, touching a canker sore that is getting bigger on her lip.

Whether it's from eating too many acidic fruits or biting your cheek, you've probably experienced a canker sore or two in your time. They're an unfortunate part of life for many people.

Most canker sores remain fairly small and resolve in a week or so. But what do you do if your canker sore keeps getting bigger?

What Is a Canker Sore?

Canker sores are a type of shallow mouth ulcer. These gray, white, or yellow lesions develop inside the lips and mouth, and sometimes on the tongue. Canker sores (sometimes called aphthous ulcers) are generally between 1–4 millimeters in diameter with a bright pink or red border.

Canker sores are extremely common and very painful, but they aren't dangerous or contagious. Many people experience a burning or stinging sensation prior to a canker sore developing. The ulcer usually hurts the most in the first few days after it appears and heals within a week or two.

If a canker sore continues to get bigger, you may need to do a little investigating to determine the cause.


Do I Actually Have a Canker Sore?

The first question to ask is whether you're actually dealing with a canker sore. Several other conditions can cause a white spot in your mouth or mimic the appearance of a canker sore. These conditions include cold sores, oral thrush, leukoplakia, and oral lichen planus.

Cold Sores

Cold sores are caused by the highly contagious herpes simplex virus, type 1 (HSV-1). HSV-1 spreads rapidly between people, triggering the formation of cold sores around the lips and mouth. Sometimes cold sores even develop inside the mouth or on the tongue.

If it is a cold sore, there are a number of treatments you can try to reduce its pain and duration.

Oral Thrush

Oral thrush is an overgrowth of Candida albicans fungus on the oral mucous membranes. Thrush isn't dangerous, but it causes some very unpleasant symptoms, including:

  • Raised white lesions that resemble cottage cheese.
  • Milky, white lesions on the gums, inner cheeks, and tongue.
  • Cottony feeling in the mouth.
  • Loss of taste.

Oral thrush is most common in infants, the elderly, and people with weakened immune systems. Using inhaled steroids and mechanical ventilation can lead to Candida overgrowth.


Leukoplakia causes white patches and sometimes open sores in the mouth. Most leukoplakia occurs on the inside of the cheeks, on the gums, underneath the tongue, or on the tongue itself. But leukoplakia doesn't cause pain.

It's important to identify leukoplakia because it's much more concerning than a canker sore. Leukoplakia can be a precursor to oral cancer. Consult with your dentist if you think you might have leukoplakia.

Oral Lichen Planus

Oral lichen planus is a chronic inflammatory condition that causes lace-like white patches to form in the mouth. Painful, red, open, swollen sores may also form.

Signs of oral lichen planus appear throughout the whole mouth. Your primary care provider can diagnose this condition, so speak to your doctor if you think you may have oral lichen planus.

Why Is My Canker Sore Getting Bigger?

Regular canker sores can get bigger for a number of reasons.

Braces and Other Oral Appliances

Braces, retainers, ill-fitting dentures, and palate expanders cause many canker sores.

Although these orthodontic appliances play an important role, they can cause painful irritation in your mouth, leading to a canker sore. If the appliance keeps rubbing against the same spot, then the canker sore doesn't get a chance to heal. Instead, the repeated friction can cause the canker sore to get bigger.

Orthodontic wax, available at your local pharmacy, can help. The wax provides a smooth, protective barrier between the rough metal and the delicate tissues in your mouth.

Chewing the Inside of Your Mouth

Many people chew on their lips or the inside of their cheeks as a form of stress relief. Sometimes they don't even realize they're doing it. If you have a canker sore that keeps getting bigger, observe yourself closely to be sure you're not repeatedly chewing on the inside of your mouth during tense times.

If you find that you do chew the inside of your mouth, try to cope with stress in other ways, such as exercise, mindfulness, enlisting the help of friends and family, and/or professional therapy.

Dietary Choices

Some foods can cause canker sores, even if your diet is well balanced and otherwise nutritious. The trouble is, these foods are different for everyone, so you'll have to determine whether a specific food is irritating your mouth and causing your canker sore to get bigger.

Some common culprits include:

  • Acidic fruits and vegetables.
  • Spicy foods.
  • Salty foods.
  • Crunchy, dense, or sharp foods.
  • Alcohol.
  • Coffee.

Additionally, foods you're allergic to can cause and irritate canker sores.


Most canker sores heal on their own within 1–2 weeks of formation. But when bacteria invade a canker sore, the continued inflammation and infection can cause the canker sore to get bigger.

Complex Canker Sores

If you've ruled out infection, chewing on your cheeks, dental appliances, and dietary triggers, what's next? You might be dealing with a complex canker sore.

Size is the most obvious difference between a regular canker sore and a complex canker sore. Regular canker sores are only a few millimeters in diameter, but complex canker sores can grow to 5 mm or even larger. They also take longer to heal — up to six weeks.

A complex canker sore may have seemingly unrelated symptoms, such as lethargy, swollen lymph nodes, and fever. Complex canker sores can indicate a more serious underlying health problem, so consult your doctor if you think you have a complex canker sore.


When Do I Need a Doctor?

If you've addressed the above causes of canker sores getting bigger to no avail, here are a few indicators that it's time to call your healthcare provider:

  • Unusually large canker sores.
  • Sores that are spreading.
  • Sores accompanied by a high fever (medically defined as above 103 degrees Fahrenheit).
  • Intense oral pain despite taking over-the-counter treatments.
  • Canker sores that last longer than three weeks.
Here we cover what to do if your canker sore keeps getting bigger and what might be causing it.

How To Prevent Future Canker Sores

Once you get to the bottom of why your canker sore keeps getting bigger, there are several steps you can take to prevent future canker sores from forming in the first place.

Avoiding the triggering foods listed above, supplementing with iron, B12, zinc, and lysine, and managing stress are a great place to start. And if you do start to develop a canker sore, light therapy like that used in the Luminance RED Mouth Sore Treatment Device has also been clinically proven to reduce pain and heal canker sores faster.

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