If you’ve ever had a painful little ulcer inside your mouth that made eating, talking, or brushing miserable, chances are you had a canker sore.
Canker sores are extremely common, affecting about 1 in 5 people, and while they can be painful and frustrating, they are not dangerous. That said, not every mouth ulcer is a canker sore — and knowing the difference matters.
This guide explains everything you need to know in simple terms.
What Is a Canker Sore?
The medical name for a canker sore is recurrent aphthous stomatitis (RAS).
A canker sore is:
- A painful ulcer inside the mouth
- Not contagious
- Not caused by a virus
- Usually round or oval, with:
- A yellow-white center
- A red, inflamed border
Canker sores almost always occur on movable, soft tissue, such as:
- Inside the cheeks
- Inside the lips
- Under the tongue
- The floor of the mouth
- The soft palate
They do not usually occur on hard, attached gum tissue or the roof of the mouth (with some exceptions).
Why Do Canker Sores Happen?
There is no single cause, but research shows canker sores are related to an immune reaction involving T-cells. In simple terms, your immune system temporarily overreacts and damages the lining of the mouth.
Common triggers include:
Immune-Related Factors
- Family history (strong genetic link)
- Stress
- Conditions affecting the immune system
- Hormonal changes
Weakened Mouth Lining
- Minor trauma (biting your cheek, sharp foods)
- Aggressive toothbrushing
- Nutritional deficiencies (iron, B vitamins, folate, zinc)
- Quitting smoking (temporarily increases risk)
Increased Irritants
- Toothpaste containing sodium lauryl sulfate (SLS)
- Certain medications (NSAIDs, beta-blockers, methotrexate)
- Foods like chocolate, coffee, cheese, gluten, strawberries, preservatives
In many people, multiple factors are involved.
Types of Canker Sores
There are three main types, and knowing which one you have helps guide treatment.
Minor Canker Sores (Most Common)
This is what most people mean when they say “canker sore.”
- Small (3–10 mm, under 1 cm)
- 1–5 sores at a time
- Very painful for their size
- Heal in 7–14 days
- Do not scar
- Occur almost exclusively on soft tissue
These tend to come and go over years.
Major Canker Sores
Less common, but more severe.
- Larger (1–3 cm)
- Deeper ulcers
- Often last 2–6 weeks
- Can heal with scarring
- May affect soft palate and throat
- Pain can be significant
These may require prescription treatment and closer monitoring.
Herpetiform Canker Sores
Despite the name, these are not herpes.
- Dozens of tiny ulcers (sometimes up to 100)
- Very frequent recurrences
- Can merge into larger irregular ulcers
- Heal in 7–10 days
- Can occur on almost any oral surface
These often look alarming but are still non-infectious.
What a Canker Sore Is NOT
Many conditions look similar to canker sores. Here’s how to tell them apart.
Cold Sores (Herpes Simplex Virus)
Not a canker sore.
Cold sores:
- Occur on keratinized tissue (lip border, hard palate, attached gums)
- Often start as blisters
- Are contagious
- Caused by a virus
- Often preceded by tingling
What to do:
- Antiviral medications may help
- Avoid touching or spreading the virus
Traumatic Ulcers
Caused by physical injury.
Common causes:
- Biting your cheek or tongue
- Sharp tooth edges
- Ill-fitting dentures
- Aggressive brushing
Clues:
- Appears exactly where trauma occurred
- Heals within 2 weeks once the cause is removed
What to do:
- Remove the source of irritation
- Use over-the-counter pain relief gels
- If it doesn’t heal in 2 weeks → get it checked
Oral Cancer (Rare but Important)
Oral cancer can sometimes look like a mouth ulcer.
Warning signs:
- Ulcer lasting more than 2 weeks
- Firm or raised borders
- Bleeding
- Numbness
- Loose teeth without gum disease
- Lump in the neck
What to do:
- Any persistent ulcer should be examined
- Biopsy may be needed to rule out cancer
Systemic Diseases Associated With Mouth Ulcers
Frequent or severe canker sores may be linked to underlying conditions such as:
- Behçet disease
- Celiac disease
- Inflammatory bowel disease
- Iron-deficiency anemia
- Vitamin deficiencies
- HIV/AIDS
- Cyclic neutropenia
In these cases, treating the underlying condition often resolves the ulcers.
What Should You Do If You Have a Canker Sore?
If It’s Mild and Infrequent
- No treatment is required
- Avoid spicy or acidic foods
- Use a soft toothbrush
- Maintain good oral hygiene
- Avoid SLS-containing toothpaste
If It’s Painful or Frequent
- Topical corticosteroid pastes or rinses
- Protective oral gels
- Prescription mouth rinses
- Tetracycline or doxycycline rinses in selected cases
- Laser therapy (in some dental offices)
Silver nitrate is no longer recommended due to risk of tissue damage.
When to See a Dentist or Doctor
- Ulcers lasting longer than 2 weeks
- Very large or deep ulcers
- Frequent recurrences
- Ulcers plus fever, genital ulcers, eye symptoms, or joint pain
- Ulcers that look unusual or don’t heal
Bottom Line
A canker sore is a common, non-contagious, painful mouth ulcer caused by an immune reaction. Most heal on their own, but recurrent or severe ulcers deserve proper evaluation to rule out other conditions.
If you’re ever unsure whether a sore is “just a canker sore,” getting it checked is always the right move.
Early evaluation brings peace of mind — and better treatment.
Disclaimer
The contents of this website, such as text, graphics, images, and other material are for informational purposes only and are not intended to be substituted for professional medical advice, diagnosis, or treatment. Nothing on this website constitutes the practice of medicine, law or any other regulated profession.
No two mouths are the same, and each oral situation is unique. As such, it isn’t possible to give comprehensive advice or diagnose oral conditions based on articles alone. The best way to ensure you’re getting the best dental care possible is to visit a dentist in person for an examination and consultation.
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