Updated September 6, 2026
Canker sores are small, painful ulcers that develop on the soft tissues inside the mouth. They usually heal on their own, but recurring, unusually large or persistent sores deserve closer attention.
One important point is often missed: not every painful spot inside the mouth is a canker sore. A sharp tooth, cheek bite, dental appliance or rough night guard edge can produce a traumatic ulcer that may look and feel similar.
What is a canker sore?
A canker sore is also called an aphthous ulcer.
These sores develop inside the mouth, commonly on the inside of the lips or cheeks, on or under the tongue, near the base of the gums or on the soft palate.
They are often round or oval with a white or yellowish center surrounded by a red border.
Unlike cold sores, canker sores are not caused by herpes simplex virus and are not contagious.
Canker sore vs. traumatic mouth ulcer
This distinction matters especially for people wearing night guards, retainers or other dental appliances.
| Feature | Canker sore / aphthous ulcer | Traumatic ulcer |
|---|---|---|
| Typical cause | Exact cause is unclear; multiple triggers and predispositions may be involved | Repeated rubbing, biting, a sharp tooth, rough dental edge or other mechanical injury |
| Location | Soft tissues inside the mouth | Often directly where the source of irritation contacts the tissue |
| Pattern | May recur even without an obvious injury | May repeatedly return in the same spot if the irritation continues |
| What helps | Symptom relief and, when severe or recurrent, professional treatment | Removing or correcting the source of friction or injury |
Appearance alone is not always enough to confidently tell one type of mouth ulcer from another.
If a sore is persistent, recurrent or unusual, a dentist or healthcare professional can evaluate it directly.
Can a night guard cause a canker sore?
A night guard can irritate the soft tissues of the mouth if an edge repeatedly rubs the cheek, lip, tongue or gum.
That irritation may create a traumatic ulcer. Oral trauma may also act as a trigger for an aphthous ulcer in someone who is already susceptible to recurrent canker sores.
What we should not assume is that every sore appearing beside a night guard is automatically a true canker sore.
Signs that the appliance itself may be contributing include:
- the sore develops exactly where an edge touches the tissue
- the same area becomes irritated each time you wear the appliance
- you can feel a rough, sharp or excessively high edge nearby
- the sore improves when the mechanical irritation is removed
If your night guard is rubbing or causing a recurrent sore spot, contact the provider rather than aggressively trimming or reshaping the appliance yourself.
See our guide to how a night guard should fit .
What causes canker sores?
There is no single proven cause of recurrent aphthous ulcers.
Current medical references describe the condition as multifactorial. Factors that may be associated with or trigger episodes include:
- minor injury or trauma inside the mouth
- emotional stress
- certain foods in susceptible people
- family or genetic predisposition
- certain medications
- some nutritional deficiencies
- certain gastrointestinal or inflammatory conditions
Mayo Clinic notes associations with low vitamin B12, zinc, folate or iron, as well as conditions such as celiac disease and inflammatory bowel disease.
That does not mean everyone with a canker sore needs extensive blood testing or supplementation.
Testing becomes more relevant when sores are severe, unusually frequent, persistent or accompanied by other symptoms.
Does dry mouth cause canker sores?
Dry mouth can make oral tissues more vulnerable to friction and irritation because saliva normally lubricates and protects the mouth.
But it is too strong to say that dry mouth directly causes most canker sores.
If you frequently have a dry mouth, discuss the cause with a dentist or healthcare professional—particularly if it is new, severe or associated with medication use or another health condition.
What are the three main types of canker sores?
Minor canker sores
Minor aphthous ulcers are the most common type. They are generally small and usually heal without scarring within about one to two weeks.
Major canker sores
Major aphthous ulcers are larger and deeper. They can be much more painful, may take several weeks to heal and can sometimes leave scars.
Herpetiform canker sores
Despite the name, herpetiform canker sores are not caused by herpes.
They consist of many very small ulcers that may occur in clusters and sometimes merge into a larger irregular sore.
How long does a canker sore last?
Most minor canker sores resolve on their own within approximately one to two weeks.
Larger major ulcers can take considerably longer.
A sore that does not heal as expected deserves professional evaluation because not every persistent mouth ulcer is a canker sore.
How can you relieve canker sore pain?
Mild canker sores usually do not require medical treatment, but several measures can make them more tolerable while they heal.
Avoid foods that irritate the area
Spicy, acidic, rough or very hot foods can sting an open ulcer and make eating uncomfortable.
Softer, bland foods are often easier to tolerate temporarily.
Use gentle oral hygiene
Continue brushing and cleaning the mouth, but avoid aggressively scrubbing directly over the ulcer.
A soft-bristled toothbrush may be more comfortable.
Try a saltwater or baking-soda rinse
Mayo Clinic includes saltwater and baking-soda rinses among self-care approaches that may reduce discomfort.
Do not swallow the rinse.
Ice may temporarily numb the area
Allowing a small ice chip to dissolve near the sore can provide temporary relief for some people.
Ask a pharmacist or clinician about pain relief
Over-the-counter topical products and pain-relieving medicines may help, depending on your age, medical history and other medications.
Follow product directions and ask a pharmacist or healthcare professional if you are unsure what is appropriate for you.
What treatments can a dentist or doctor prescribe?
More severe or recurrent aphthous ulcers may warrant prescription treatment.
Topical corticosteroids are among the main treatments used for recurrent aphthous stomatitis. Depending on the situation, a clinician may use a paste, rinse or another topical preparation.
Treatment should be individualized because other oral conditions—including infections—can sometimes resemble aphthous ulcers.
Antibiotics should not be presented as a routine treatment for an uncomplicated canker sore.
Should you use a mouth guard to treat canker sores?
A night guard is not a treatment for aphthous ulcers themselves.
However, if repeated cheek or tongue biting is continually injuring the same tissue, reducing that mechanical trauma may help prevent re-injury while the area heals.
That is a different goal from treating recurrent aphthous stomatitis.
If repeated cheek biting is the problem, read: How to Stop Biting the Inside of Your Cheek .
What if your night guard is rubbing an existing sore?
Continuing to rub an already injured area can keep the tissue irritated.
If the edge of an appliance repeatedly contacts a sore:
- stop forcing the appliance against the painful area
- check whether the guard is fully seated
- look for a rough or unusually extended edge
- contact the dentist or provider that made the appliance
Do not use boiling water, aggressive trimming or household tools to reshape a custom night guard.
If the guard is also causing tooth pressure or pain, see: Why Does My Night Guard Hurt My Teeth?
When should you see a dentist or doctor for a mouth sore?
Most minor canker sores heal without treatment, but professional evaluation becomes more important when a sore behaves differently than expected.
Contact a dentist or healthcare professional if you have:
- a mouth sore lasting about two weeks or longer
- an unusually large or deep ulcer
- frequent sores or new ulcers appearing before old ones heal
- pain that is difficult to control
- difficulty eating or drinking because of the sore
- a high fever accompanying mouth sores
- a lesion that repeatedly develops in exactly the same location
- other unexplained symptoms elsewhere in the body
Seek urgent care for significant facial or neck swelling, difficulty swallowing or difficulty breathing.
Frequently asked questions
Are canker sores contagious?
No. Canker sores are not contagious and are different from herpes-related cold sores.
Can stress cause canker sores?
Stress is considered a possible trigger for recurrent aphthous ulcers in susceptible people, but it is not the sole cause.
Can biting your cheek cause a canker sore?
Oral trauma may trigger an aphthous ulcer in someone who is susceptible. A cheek bite can also create a traumatic ulcer that is not technically a canker sore.
Can a night guard cause mouth sores?
Yes. A poorly fitting or rough appliance can create friction or pressure against the soft tissues and cause a traumatic sore. Recurrent irritation in the same location should prompt a fit evaluation.
Should you stop wearing a night guard if it rubs a sore?
Do not continue forcing an appliance against a painful or injured area. Contact the provider so the fit and trim line can be evaluated.
Is a canker sore the same as a cold sore?
No. Canker sores develop inside the mouth and are not caused by herpes. Cold sores are associated with herpes simplex virus and often develop on or around the lips.
Do canker sores need antibiotics?
Usually not. Uncomplicated aphthous ulcers are not bacterial infections. Treatment is generally aimed at symptom control and reducing inflammation.
The bottom line
Canker sores are common, painful ulcers inside the mouth, and most minor sores heal within one to two weeks.
Their exact cause is not fully understood. Trauma, stress and several medical or nutritional factors may influence susceptibility, but not every mouth sore is an aphthous ulcer.
If a sore repeatedly appears exactly where a night guard or another dental appliance rubs, consider a mechanical irritation or traumatic ulcer and have the appliance evaluated.
Persistent, unusually large, recurrent or severe mouth sores should be evaluated by a dentist or healthcare professional rather than repeatedly self-treated.
Sources
Mayo Clinic. Canker Sore: Symptoms and Causes and Canker Sore: Diagnosis and Treatment.
Merck Manual Professional Edition. Recurrent Aphthous Stomatitis. Reviewed March 2026.
Merck Manual Professional Edition. Stomatitis.
Editor's note: Updated September 6, 2026. This article was substantially revised to distinguish recurrent aphthous ulcers from traumatic mouth sores, clarify the role of oral trauma and dental-appliance irritation, update treatment and red-flag guidance, remove outdated product grids and pricing, and remove unsupported claims about antibiotics, lasers and oral appliances healing canker sores.
This article provides general educational information and is not a substitute for individualized dental or medical care.