A small sore inside your mouth can cause a surprising amount of pain. It may sting when you drink orange juice, hurt when you brush your teeth, make eating uncomfortable, or seemingly appear overnight for no obvious reason. Then, after a week or two, it disappears. Until another one shows up.

These painful ulcers are commonly called canker sores, or aphthous ulcers. When they occur repeatedly, the condition is often called recurrent aphthous stomatitis (RAS). The frustrating part is that there usually isn’t one simple answer to the question patients ask me most:

“Why do I keep getting them?”

Current research suggests recurrent canker sores are multifactorial. Genetics, immune activity, minor trauma, nutritional deficiencies, stress, certain foods, medications, and underlying medical conditions may all play a role in susceptible individuals.

A recent 2026 review describes recurrent aphthous stomatitis as involving an inappropriate cell-mediated immune response in genetically predisposed individuals, with factors such as nutritional deficiencies, psychological stress, microtrauma, and food intolerances potentially triggering episodes. PubMed

“When a patient frequently develops canker sores, I don’t automatically assume there is one specific food, vitamin deficiency, or dental problem causing them. The pattern of the ulcers often tells us much more than any single episode.”

— Marielaina Perrone, DDS

What Exactly Is a Canker Sore?

A typical canker sore is a small, painful ulcer that develops on the soft tissues inside the mouth. It may occur inside the lips or cheeks, underneath or along the tongue, near the base of the gums, or on the soft palate. Most have a round or oval appearance with a white, yellowish, or gray center surrounded by a red inflammatory border. Some people notice tingling or burning before the ulcer becomes visible. Mayo Clinic

The most important distinction is this:

Canker sores are not cold sores.

Cold sores are caused by herpes simplex virus and commonly develop on or around the lips. Typical canker sores develop inside the mouth and are not caused by herpes and are not contagious. Mayo Clinic

That means you cannot catch a typical canker sore from kissing someone, sharing utensils, or drinking from the same glass.

Why Do Canker Sores Happen?

For many people, we never identify one definitive cause. That’s why I prefer to think about susceptibility plus triggers rather than looking for one universal culprit.

A person may have an underlying genetic or immune susceptibility, and then something (stress, trauma, nutritional changes, illness, or another trigger) helps initiate an episode.

Research increasingly supports an immune-mediated component to recurrent aphthous stomatitis, although its exact pathogenesis remains incompletely understood. PMC

Here are some of the factors we consider.

1. Minor Trauma Can Trigger a Sore

Sometimes the explanation is remarkably simple.

You bite the inside of your cheek.

A toothbrush slips.

A sharp piece of food scratches the tissue.

A rough tooth or dental restoration repeatedly irritates the same location.

Dental treatment or an orthodontic appliance may temporarily traumatize the mucosa.

In someone predisposed to aphthous ulcers, relatively minor tissue injury can help trigger a lesion. PMC

This is also why location matters. If an ulcer keeps appearing in exactly the same place, I want to look for a local source of irritation rather than simply calling it another canker sore.

2. Stress Really Can Matter

Many patients notice their canker sores appearing during particularly stressful periods.

College exams.

Work deadlines.

Travel.

Family stress.

Lack of sleep.

Major life changes.

Psychological stress has repeatedly been identified as a possible precipitating factor in recurrent aphthous stomatitis, although it isn’t necessarily the underlying cause of the condition. PMC In other words, saying “stress can trigger my canker sores” is reasonable.

Saying “stress is the cause of all canker sores” isn’t.

“Patients often know their own patterns remarkably well. If someone tells me their ulcers consistently appear during stressful weeks, I take that seriously. Our job is to look at the entire pattern rather than dismissing it because there isn’t one simple laboratory test for canker sores.”

— Dr. Marielaina Perrone, DDS

3. Nutrition May Play a Role

Recurrent oral ulcers can sometimes be associated with deficiencies involving nutrients such as:

• Iron
• Vitamin B12
• Folate
• Zinc

That does not mean everyone who develops a canker sore should immediately start taking multiple supplements. The more important question is whether there is a reason to suspect a deficiency.

For someone with unusually frequent, severe, persistent, or newly developing recurrent ulcers, nutritional or hematologic evaluation may sometimes be appropriate. Reviews of recurrent aphthous stomatitis recognize hematinic and nutritional deficiencies among potential contributing or secondary factors. PubMed

Testing first is generally more meaningful than blindly treating every canker sore with vitamins.

4. Certain Foods May Trigger Ulcers in Some People

Patients frequently blame acidic or spicy foods for canker sores. The relationship isn’t always straightforward.

Foods such as citrus fruits, tomatoes, spicy foods, chocolate, nuts, and other foods have been reported as possible triggers in susceptible individuals. Mayo Clinic

But there is an important distinction. An acidic food may irritate an ulcer that already exists without necessarily having caused it.

Anyone who has accidentally put orange juice on an active canker sore understands that distinction immediately.

If you repeatedly notice the same food preceding an outbreak, keeping a simple diary can sometimes reveal a pattern. I wouldn’t recommend eliminating large groups of nutritious foods without a consistent reason.

5. Could Your Toothpaste Be Involved?

This one surprises many patients. Some research and clinical guidance have considered sodium lauryl sulfate (SLS), a foaming ingredient found in some toothpastes—as a possible trigger or aggravating factor in susceptible people.

Mayo Clinic includes toothpastes and mouth rinses containing SLS among possible canker-sore triggers and suggests avoiding them when recurrent ulcers appear to be a problem. Mayo Clinic

That doesn’t mean SLS causes canker sores in everyone. But for someone experiencing frequent recurrent ulcers, trying an SLS-free toothpaste can be a simple experiment worth discussing with a dentist.

6. Genetics May Help Explain Why They Run in Families

Some people seem naturally more susceptible to recurrent aphthous ulcers. And sometimes several family members have the same problem.

Research supports a genetic component to recurrent aphthous stomatitis, and immune-related genetic differences are among the factors being investigated. PMC

That may help explain why one person repeatedly develops ulcers after minor trauma or stress while another person rarely develops one at all. It isn’t necessarily that one person is brushing incorrectly. Their biological susceptibility may simply be different.

7. Hormonal Changes May Influence Some Patients

Some women report changes in canker-sore frequency around menstruation or other hormonal changes. Hormonal influences have been proposed among the many factors that may modify aphthous-ulcer susceptibility, although they do not explain the condition in every patient. Mayo Clinic

Again, the key is the pattern. A single canker sore doesn’t tell us very much. Repeated episodes occurring under similar circumstances may.

8. Certain Medications Can Cause Aphthous-Like Ulcers

Not every ulcer that looks like a canker sore is necessarily classic recurrent aphthous stomatitis. Some medications can be associated with oral ulceration or aphthous-like reactions. NSAIDs and certain other medications have been reported among potential triggers or causes. Mayo Clinic

If recurrent mouth ulcers begin after starting or changing a medication, tell both your dentist and prescribing physician.

Do not stop a prescribed medication on your own because you suspect it may be involved. The medication may be unrelated, or the benefit of the medication may far outweigh the oral side effect.

When Canker Sores May Point to Something Else

This is the part of the conversation I believe deserves more attention. Most occasional canker sores in otherwise healthy people are benign and self-limiting. But frequent, severe, unusual, or newly developing recurrent ulcers deserve a closer look.

Aphthous-like ulcerations can sometimes occur in association with systemic conditions such as:

Celiac disease

Crohn’s disease

Ulcerative colitis

Behçet’s disease

Certain immune deficiencies

Hematologic abnormalities

Nutritional deficiencies

The differential diagnosis can be particularly important when recurrent oral ulcers are accompanied by symptoms elsewhere in the body. PubMed

That doesn’t mean someone with recurring canker sores should assume they have an autoimmune or gastrointestinal disease.

Most don’t. But the mouth can occasionally provide an early clue that deserves investigation.

“An isolated canker sore is usually just an isolated canker sore. What gets my attention is when the pattern changes. When ulcers become unusually frequent, larger, slower to heal, or begin occurring along with symptoms somewhere else in the body.”

— Marielaina Perrone, DDS

Three Types of Canker Sores

Not all aphthous ulcers behave the same way.

Minor Aphthous Ulcers

These are by far the most common. They are relatively small, usually heal within about one to two weeks, and generally do not scar. Mayo Clinic

Major Aphthous Ulcers

Major ulcers are larger and deeper.They can be considerably more painful, may take several weeks to heal, and can sometimes leave scarring. Mayo Clinic

These deserve greater attention, particularly when they recur.

Herpetiform Aphthous Ulcers

Despite the confusing name, herpetiform aphthae are not herpes. They appear as numerous very small ulcers that may cluster together and merge into larger irregular lesions. Mayo Clinic

What Can You Do When You Have a Canker Sore?

For a typical minor canker sore, treatment is largely about making the area more comfortable while it heals. Most resolve spontaneously within one to two weeks. Mayo Clinic

During that time:

Avoid foods that noticeably irritate the ulcer.

Use a soft toothbrush.

Avoid repeatedly touching or traumatizing the area.

Be careful with particularly acidic, salty, spicy, or sharp foods if they cause discomfort. For more painful or recurrent cases, dentists and physicians may recommend topical medications designed to reduce inflammation or discomfort. More severe recurrent aphthous disease can require additional medical management. PubMed

The right treatment depends on whether we’re treating an occasional nuisance or a recurrent disease pattern.

Should You Put Salt on a Canker Sore?

I wouldn’t recommend rubbing salt directly into an ulcer. Although saltwater rinses are commonly used as a gentle oral rinse, putting concentrated salt directly on exposed ulcerated tissue can be extremely painful and unnecessarily irritating.

More pain doesn’t mean better healing. The goal should be to protect the tissue and control symptoms while healing occurs.

Should You Pop a Canker Sore?

No. A canker sore isn’t a pimple or blister filled with material that needs to be drained.

It is an ulcer, an area where the surface lining of the oral tissue has been lost. There is nothing useful to pop. Trying to puncture or manipulate it simply creates additional trauma.

When Should a Mouth Sore Be Examined?

This is perhaps the most important part of the article. People sometimes assume every sore inside the mouth is a canker sore. It isn’t.

Trauma, infections, immune disorders, medication reactions, inflammatory diseases, and other oral conditions can produce ulcerations. And persistent oral ulcers can occasionally represent something much more serious.

Mayo Clinic recommends professional evaluation for unusually large sores, sores that recur frequently, significant pain, difficulty eating or drinking, or ulcers lasting two weeks or longer. Mayo Clinic

From a dental standpoint, I am particularly interested in an ulcer that is:

Not healing normally

Persisting beyond approximately two weeks

Becoming larger

Unusually firm or irregular

Bleeding without explanation

Frequently recurring

Always appearing in the same location

Accompanied by fever, gastrointestinal symptoms, skin lesions, genital ulcers, weight loss, swollen lymph nodes, or other systemic symptoms

Newly appearing repeatedly in an adult who never previously experienced canker sores Those findings don’t automatically mean something serious is wrong. They mean the lesion deserves to be properly evaluated rather than repeatedly self-diagnosed.

Canker Sore or Oral Cancer?

This is an understandable concern, particularly for someone who has read about persistent mouth ulcers as a warning sign of oral cancer.

A typical minor canker sore is painful, develops on movable soft tissue inside the mouth, and heals within roughly one to two weeks.

A suspicious lesion may behave differently. But appearance alone is not always enough to make the distinction. That’s why persistence matters.

If an unexplained ulcer doesn’t heal as expected, don’t continue treating it as a canker sore indefinitely.

Have it examined.

“One of the simplest rules I give patients is this: your mouth heals remarkably quickly. If something isn’t healing the way you expect it to, give us the opportunity to look at it.”

— Dr. Marielaina Perrone, DDS

Why Your Dentist May Ask Questions That Don’t Seem Dental

If you’re seeing me because of recurring mouth ulcers, I may ask about things that seem unrelated to dentistry.

Have you recently changed medications?

Do you have gastrointestinal symptoms?

Have you experienced unexplained fatigue?

Have you changed your diet?

Are the ulcers associated with stress?

Does anyone else in your family get them?

Do they always occur in the same place?

Have you noticed skin, eye, or other symptoms?

How long does each ulcer last?

When did recurrent episodes begin?

Those questions help distinguish ordinary recurrent aphthous stomatitis from an ulcer that may be secondary to something else. The mouth is part of the body. Sometimes understanding what is happening in the mouth requires looking beyond it.

Frequently Asked Questions About Canker Sores

Are canker sores contagious?

No. Typical aphthous ulcers are not contagious and are not caused by herpes simplex virus. Mayo Clinic

Why do I suddenly keep getting canker sores?

Possible triggers include stress, minor oral trauma, nutritional deficiencies, certain foods, medication effects, immune changes, and systemic conditions. In many people, however, no single cause can be identified. PubMed

Can vitamin deficiencies cause canker sores?

Deficiencies involving iron, vitamin B12, folate, and zinc have been associated with aphthous-like or recurrent oral ulcers in some patients. Frequent or severe ulcers may warrant further evaluation rather than automatically taking supplements. Mayo Clinic

Is a canker sore the same as herpes?

No. Cold sores are associated with herpes simplex virus. Canker sores occur inside the mouth and are not caused by herpes. Mayo Clinic

Can stress cause canker sores?

Stress is recognized as a potential trigger in susceptible individuals, although it isn’t considered the single underlying cause of recurrent aphthous stomatitis. PMC

How long should a canker sore last?

Typical minor canker sores usually heal within approximately one to two weeks. Larger major aphthous ulcers can last considerably longer. Mayo Clinic

When should I worry about a canker sore?

An unusually large, severe, recurrent, or persistent ulcer deserves professional evaluation. A sore that hasn’t healed after approximately two weeks should not simply be assumed to be another canker sore. Mayo Clinic

The Bigger Lesson: Look for the Pattern

Most canker sores aren’t dangerous. But they’re also a good example of why dentistry shouldn’t be limited to looking for cavities.

Your oral tissues can respond to trauma, nutrition, immune activity, medications, systemic disease, stress, and countless other biological influences. When a canker sore appears once or twice and disappears normally, there may be little mystery worth solving. When it happens repeatedly, however, I start looking for the pattern.

Where does it occur?

How long does it last?

What happened before it appeared?

Has anything changed?

Are there symptoms elsewhere in the body?

Sometimes we identify a trigger.

Sometimes we discover a dental source of irritation.

Sometimes medical testing is appropriate.

And sometimes the answer is simply that a person is susceptible to recurrent aphthous stomatitis.

“Not every canker sore needs an investigation. But when your mouth repeatedly tells us the same story, it’s worth listening.”

— Marielaina Perrone, DDS

At my Henderson, Nevada dental practice, serving Green Valley and the greater Las Vegas Valley, examining the soft tissues of the mouth is part of comprehensive dental care. Not something separate from it. Because sometimes a small sore is simply a small sore. And sometimes understanding why it keeps coming back is the more important question.

Medically Reviewed by Marielaina Perrone, DDS

Dr. Marielaina Perrone, DDS, is a highly experienced cosmetic, restorative, implant, and family dentist serving Henderson, Green Valley, and the greater Las Vegas area. A graduate of the Stony Brook University School of Dental Medicine, she has more than 25 years of clinical experience in preventive dentistry, periodontal care, restorative dentistry, and oral-systemic health. Through evidence-based, personalized care, Dr. Perrone helps patients manage complex medical conditions—including Sjögren’s syndrome—that can affect long-term oral health and overall well-being.

Updated: October 2026

Research & Patient Resources

2026 Review: Idiopathic and Secondary Recurrent Aphthous Stomatitis

Review: Essentials of Recurrent Aphthous Stomatitis

Review: Etiology, Deficiencies and Management of Recurrent Aphthous Stomatitis

Mayo Clinic: Canker Sore Causes and Symptoms