Canker sores: why they form and what actually helps
A spot smaller than a pencil eraser can ruin a week of eating and talking. Here is what a canker sore really is, the triggers that stack up to cause one, and the short list of things that genuinely shorten the misery.
A canker sore is a harmless aphthous ulcer, not a cold sore, and not contagious. It shows up when triggers stack: minor trauma, SLS in toothpaste, nutrient gaps, stress, and acidic food. Most heal in 7 to 14 days. Recurrent aphthous stomatitis affects up to 1 in 5 people. Treat the one you have gently with a salt water rinse, a barrier gel, and no citrus. To get fewer of them, the single best supported change is dropping SLS toothpaste. Check ferritin, B12, and folate if they keep coming back monthly.
This article is by Minvelle, an Austrian oral-care brand. It stands on its own. If you are doing a bigger oral-health audit, the 60-second enamel check sorts what your enamel actually needs.
What is actually happening in the tissue when a sore forms?
It helps to know that a canker sore is not a hole that something burned into your mouth. It is your own immune system attacking a small patch of your own lining. The clinical reviews are consistent on this: recurrent aphthous stomatitis is best understood as a T-cell-mediated immune reaction, and the central messenger driving the damage is a signalling protein called tumour necrosis factor-alpha, usually shortened to TNF-alpha. Everything that makes a sore look and feel the way it does follows from that one fact.
Here is the rough sequence, drawn from the pathogenesis literature summarised in StatPearls and a dedicated immunology review in Archivum Immunologiae et Therapiae Experimentalis. Some trigger, which can be a cheek bite, a detergent stripping the mucous layer, or a shift in the local environment, exposes the keratinocytes that make up the surface of your mouth lining. Those cells start releasing alarm signals. T-cells and other immune cells flood in, and they pump out a cluster of inflammatory cytokines that the immunology review found elevated in affected tissue, including interleukin-2, interferon-gamma, and TNF-alpha. TNF-alpha then does the visible work: it tells the lining of nearby small blood vessels to become sticky and recruits a wave of neutrophils to the spot. Those neutrophils, plus CD8 killer T-cells, destroy the surface layer of cells, and what you see in the mirror is the small crater they leave behind.
This is why the white or yellow centre of a sore is not pus from an infection. It is a thin layer of fibrin and dead cells, a kind of natural scab over raw tissue, with the red rim being the inflamed, blood-rich border around it. There is no single germ to kill, which is exactly why antibiotics and antiviral creams do nothing for a true canker sore. You are not fighting an invader. Your immune system has, briefly and locally, turned on a small piece of you.
The cytokine picture also explains a few things that puzzle people. A 2024 systematic review and meta-analysis in PLoS One pooled the cytokine studies and found that people with active sores tend to run higher levels of the pro-inflammatory signals that drive the attack, especially interleukin-2, interleukin-6, and TNF-alpha, and lower levels of interleukin-10, one of the body's main brakes on inflammation. In plain terms, a canker-prone mouth seems to be quicker to start an inflammatory response and slower to switch it off. That tilt is part of why some people sail through life with a sore or two a decade while others get them in clusters: the threshold for tipping into that self-attack is simply lower in some of us, and research suggests it has a real genetic component, since sores often run in families.
It is worth being precise about what the immune framing does and does not mean, because the word immune gets people reaching for the wrong shelf. A canker sore is not an allergy in the classic sense, and it is not an autoimmune disease in the way that something like lupus is. The clinical reviews describe it as a focal, self-limiting immune reaction: the response is real, it is aimed at your own tissue, but it switches off on its own and leaves no lasting damage in the ordinary minor type. That is why the sore heals cleanly without scarring in a week or two, and why nothing you put on it needs to defeat a lurking pathogen. The job is simply to keep the area calm while your own regulation, including that interleukin-10 brake the meta-analysis found running low, catches up and shuts the flare down.
The same biology quietly debunks a long list of folk fixes. Because there is no germ, dabbing the sore with strong antiseptics, alcohol, or undiluted essential oils does not attack a cause, it just adds a chemical insult to tissue that is already inflamed, which tends to sting more and can slow things down. Cauterising agents that burn the sore are a separate category: they may briefly reduce pain by killing the nerve endings at the surface, but they are destroying tissue to do it, and the research base behind routine use is thin. Knowing the mechanism is the cheapest filter you have for this kind of advice. If a remedy is sold on the idea of disinfecting or burning out an infection, it is solving a problem your mouth does not have.
Two practical points fall out of the biology. First, because TNF-alpha sits at the centre of the process, the most severe and stubborn cases are exactly the ones that respond to drugs that block it, which is why TNF-alpha inhibitors are reserved for the rare, debilitating end of the spectrum rather than the sore you get before a big presentation. Second, because the damage is inflammatory rather than infectious, the at-home goal is never to disinfect the sore into submission. It is to lower irritation and let the inflammatory flare burn out on its schedule. That reframing changes what you reach for, which is the subject of the next section.
What the research actually shows about treatments
There is no shortage of products that promise to make a canker sore disappear, and most of the honest evidence sits somewhere between modest and unimpressive. That is not a reason to do nothing. It is a reason to know what each option realistically buys you, so you spend money and effort on the things that actually move the timeline. The most useful single source here is a 2022 network meta-analysis in the journal Medicina that pooled 72 randomised controlled trials covering 5,272 people and 29 different topical treatments, then ranked them. Below is how the credible options stack up.
The honest summary of that table is that the topical options compete for second place behind your own healing. They reliably reduce pain and shave a little off the duration, but none of them is a switch that ends an outbreak on contact. The dental practitioners' treatment guideline follows the same logic: start with the gentlest measures, identify and fix anything correctable such as a nutrient gap or an allergy, reach for a topical corticosteroid for the painful ones, and only escalate to systemic, immune-modulating drugs for the rare, severe, relentless cases. That last tier is deliberately small, because the trade-offs of suppressing the whole immune system are not worth it for a sore that was going to heal in two weeks anyway.
One nuance the network meta-analysis underlines is the gap between statistical ranking and everyday usefulness. A treatment can edge out the others in a pooled model and still make a difference you would struggle to notice, because the baseline is a wound that heals itself either way. The trials measure things like days to healing, ulcer size, and pain scores, and the gains for most topical options are measured in a day or two and a few points of comfort, not in making the sore disappear. That is not a knock on the research, it is the honest ceiling of the condition. It also means the smart way to read the table is by downside, not just upside: a barrier gel or a salt-water rinse sits near the top of the risk-free options, so they are sensible defaults even when their measured effect is modest, whereas anything that stings, burns, or carries side effects has to clear a higher bar to be worth it for a self-healing sore.
The deficiency angle, and when it is worth chasing
If you get sores often, the most useful question is not which gel to buy but whether something measurable is keeping your tissue on a hair trigger. The deficiency link is the part of canker-sore research with the cleanest practical payoff, because unlike stress or genetics, a low level of iron, folate or vitamin B12 is something a blood test can find and a pill can correct. The catch is that this only helps the subset of people who are actually low, which is why guessing with random supplements is usually wasted money.
Vitamin B12 has the most striking single trial behind it. A randomised, double-blind, placebo-controlled study published in the Journal of the American Board of Family Medicine gave people prone to recurrent ulcers a daily sublingual dose of B12 for six months. By the final months, the treated group had markedly fewer ulcers and far less pain than the placebo group, and a much larger share reached stretches with no sores at all. The result that surprised people was that the benefit did not depend on starting B12 level: even patients whose blood B12 looked normal improved. That is a hint the relationship is more complicated than a simple deficiency, and the authors themselves framed it as a low-risk, inexpensive option worth trying rather than a sure thing.
There is one deficiency-adjacent pattern worth flagging because it occasionally points to something bigger. In children and adults with coeliac disease, recurrent mouth ulcers turn up far more often than in people without it. A study in the Pakistan Journal of Medical Sciences found recurrent aphthous lesions in a large share of children with coeliac disease and in none of the matched controls, and the authors suggested that frequent unexplained ulcers in a child should prompt thinking about coeliac disease rather than being shrugged off. The likely mechanism loops back to the same nutrients: untreated coeliac disease impairs absorption of iron, folate and B12, the exact gaps tied to sores elsewhere. So persistent ulcers alongside gut symptoms, fatigue or poor growth are a reason to ask a doctor about more than the sores themselves.
The takeaway is measured, not magical. If your sores are frequent, ask for iron, ferritin, folate and B12 to be checked, fix anything that is genuinely low, and give it a couple of months. If everything comes back normal, more supplements are not the answer, and the lever shifts back to the triggers you can control, especially the foaming detergent in conventional toothpaste covered earlier on this page.
When recurrent sores are a clue to something systemic
The vast majority of recurrent canker sores are exactly what they look like: an over-eager local immune response with no wider meaning. But because the mouth is one of the first places systemic inflammation shows itself, a small minority of stubborn, severe or unusually patterned ulcers are the body's early warning of a condition that lives well beyond the mouth. Knowing the shape of those exceptions tells you when a sore is worth a doctor's attention rather than a barrier gel.
The best-known example is Behcet's disease, an inflammatory condition in which recurrent mouth ulcers sit alongside genital ulcers, eye inflammation and skin lesions. A 2025 systematic review in Medicina Oral, Patologia Oral y Cirugia Bucal pulled together studies of people who present with recurrent aphthous ulceration and asked how many turn out to have Behcet's. The reassuring headline is that the share is generally low, with most individual studies landing in the low single digits, though it climbs in countries along the old Silk Road from the Mediterranean to the Far East. In plain terms, mouth ulcers are a near-universal feature of Behcet's, but the reverse is not true: having recurrent canker sores almost never means you have Behcet's. The signal to watch for is sores arriving in company, particularly with eye or genital symptoms.
A second pattern shows up at the other end of life. When recurrent ulcers start very early in childhood and come with fevers or other signs of body-wide inflammation, the cause is occasionally a single faulty gene in the immune system rather than the ordinary trigger stack. A study in the Pediatric Rheumatology Online Journal sequenced children who had recurrent aphthous stomatitis together with systemic inflammation and found that several carried rare pathogenic mutations in immune-regulating genes, with disease that had typically begun before the age of six. One child's mother carried the same mutation, a clean illustration of how this can run in families. The practical point is narrow but real: most kids with the odd mouth ulcer are completely fine, but very early onset plus recurring fevers is a reason to look deeper rather than to keep treating the surface.
None of this should make you anxious about an ordinary sore. The whole point of mapping the exceptions is to make the common case feel boring again. If your ulcers heal inside about three weeks, stay confined to your mouth, and do not travel with fever, rashes, eye trouble or gut symptoms, the systemic conditions above are extremely unlikely. It is the pattern, not the single sore, that earns a closer look: relentless recurrence, ulcers larger than a centimetre, anything past three weeks, or sores that show up with symptoms elsewhere in the body. Those are the cases to take to a dentist or doctor, and the reason the timeline rule earlier on this page is worth keeping in your back pocket.
Putting it together
Step back and the biology tidies up the whole topic. A canker sore is a brief, local immune flare with TNF-alpha at its centre, not an infection, so disinfecting it harder is the wrong instinct. The honest treatment evidence says the topical options reduce pain and trim the timeline a little, with a corticosteroid the strongest of a modest field, and that the real fixes are the correctable triggers: a detergent toothpaste, a genuine nutrient gap, the stress and trauma that lower your threshold. The rare systemic causes have their own tell, which is sores that travel with other symptoms or start unusually early. Match the response to where your case actually sits on that map, and a canker sore goes back to being what it is for almost everyone, a small nuisance that heals itself within a couple of weeks.
As always, this is general information about what the research suggests, not a diagnosis or a treatment plan for your specific situation. If a sore is severe, keeps coming back, lasts beyond three weeks, or arrives with symptoms elsewhere in your body, see a dentist or doctor who can examine you in person.
A canker sore is a harmless mouth ulcer, not a cold sore and not contagious. It shows up when triggers stack: minor trauma, the SLS detergent in many toothpastes, nutrient gaps, stress, and acidic food. Most heal in 7 to 14 days on their own.
Treat the one you have gently (salt-water rinse, barrier gel, no citrus). To get fewer of them, the single best-supported change is dropping SLS toothpaste.
Few mouth problems feel as out of proportion to their size as a canker sore. A lesion smaller than a pencil eraser can make eating, talking, and even smiling miserable for a week. The reassuring part is that almost all of them are harmless and, once you understand what sets them off, fairly predictable. This guide covers what they actually are, why they keep coming back, and the short list of things that genuinely help, including the one trigger most people never think to change.
What is a canker sore, exactly?
A canker sore (the clinical term is aphthous ulcer) is a shallow, round lesion on the soft tissue inside your mouth. It usually has a white or yellow centre with a red border, and it turns up on the inner cheeks, the tongue, or the soft tissue near the gum line. It is not an infection in the usual sense, and there is no single germ to blame.
Most people get the minor type: lesions under a few millimetres that heal on their own in seven to fourteen days without scarring. A smaller group gets the major type, larger or clustered ulcers that take longer to settle and can occasionally leave a mark. Recurrent aphthous stomatitis, the medical name for getting them over and over, affects up to one in five people at some point, according to reviews in the British Dental Journal.
Canker sore or cold sore: how do you tell?
People use the two names interchangeably, but they are completely different problems with different causes. Getting this right matters, because one is contagious and one is not.
Why do canker sores form?
There is rarely one cause. Canker sores tend to appear when several triggers line up at once, which is why the same person can go months without one and then get two in a fortnight during a rough patch. These are the triggers that show up most consistently in the research.
A cheek bite, a stiff toothbrush, sharp food like crisps, or an ill-fitting dental appliance can break the surface and start an ulcer.
The foaming detergent in many conventional toothpastes. Trials found people who switched to an SLS-free paste had fewer and shorter outbreaks. SLS is thought to strip the protective mucous layer and leave tissue exposed.
Low iron, folate, zinc, and vitamin B12 show up repeatedly in people with recurrent sores. Correcting a real deficiency reduces outbreaks.
Outbreaks often cluster around exams, deadlines, and the menstrual cycle. Stress does not create a sore on its own, but it lowers the bar for the others.
Citrus, tomatoes, vinegar, and coffee do not cause sores, but they sharply irritate ones that have already started.
Because the triggers stack, the most useful thing you can do is track them. A new toothpaste plus a stressful week plus poor sleep is a classic combination. Note when sores appear for a couple of months and a pattern almost always surfaces.
Does your toothpaste trigger canker sores?
This is the trigger most people never check, and it is the one with the most direct trial support. Sodium lauryl sulfate is the surfactant that makes toothpaste foam. It has no role in cleaning teeth; it is there for the lather. In a series of crossover trials in the 1990s (Herlofson and Barkvoll, Acta Odontologica Scandinavica), people prone to recurrent ulcers reported significantly fewer sores during the months they used an SLS-free paste, and the sores they did get were less painful.
The proposed mechanism is simple: SLS is a detergent, and detergents disrupt the thin mucous film that protects the inside of your mouth. Strip that layer twice a day and tissue that was already prone to ulceration gets a head start. If your toothpaste foams up into a mouthful of suds, it almost certainly contains SLS, and switching is one of the cheapest experiments in oral care.
The opposite is often true. Aggressive, over-frequent brushing with a stiff brush is a trauma trigger, and the foaming detergent that signals a "deep clean" can be part of the problem. Gentle and consistent beats hard and frequent.
What actually helps a canker sore heal?
Nothing makes a canker sore vanish overnight, because healing is your tissue's job and it takes its time. What you can do is keep the wound clean, protected, and un-irritated so it settles faster and hurts less along the way.
About half a teaspoon of salt in a cup of warm water, two or three times a day. Cheap, gentle, and it keeps the area clean.
A protective gel, or a mild corticosteroid paste from a pharmacist, cuts pain and shortens the episode. Choose alcohol-free rinses so you are not stinging the lesion.
No crisps, citrus, vinegar, or very hot drinks directly on the spot until it closes over.
How do you stop them coming back?
Prevention is about removing the triggers you can control and closing the gaps you can measure. Four moves do most of the work.
Drop the SLS. If you get sores more than a few times a year, switch to an SLS-free toothpaste. This is the one change with real trial support behind it.
Brush softer. A soft-bristled brush and a slower hand remove a common, entirely avoidable trauma trigger.
Track the pattern. A couple of months of notes usually reveals a specific food, a stress phase, or a point in the cycle.
Check the bloodwork. If sores are frequent, ask your doctor to test iron, B12, and folate rather than guessing.
Minvelle is a gum, so there is no SLS to strip your mouth
Minvelle is a remineralizing chewing gum, not a toothpaste, so it contains none of the foaming detergent linked to recurrent ulcers. Between outbreaks, sugar-free gum stimulates saliva, your mouth's natural buffer, and adds xylitol and Chios mastic. It is not a treatment for an ulcer you already have, and no gum is. The real lever for canker-prone mouths is dropping SLS toothpaste; the gum just fits a low-irritant routine.
See the formula →When is a mouth ulcer something more serious?
The vast majority of canker sores are annoying rather than dangerous. But the timeline is a clean rule worth remembering, because a small minority of mouth ulcers point to something that needs a closer look.
A sore lasts longer than three weeks, keeps returning in the exact same spot, grows larger than a centimetre, or arrives alongside fever, a rash, or gut symptoms. Persistent ulcers can occasionally signal coeliac disease, inflammatory bowel disease, or, rarely, something that warrants a biopsy. Three weeks and gone is fine. Three weeks and lingering is a checkup.
Frequently asked questions
Are canker sores contagious?
No. Canker sores are not caused by a virus and cannot spread between people or to other parts of your body. That is the key difference from cold sores, which are caused by the herpes simplex virus, appear on the lips, and are contagious.
How long does a canker sore last?
Most minor canker sores heal on their own in seven to fourteen days without scarring. Larger or clustered ulcers can take longer. Gone within about three weeks is normal; lingering past three weeks is worth a checkup.
Can toothpaste cause canker sores?
It can be a trigger. Several trials found people prone to recurrent ulcers had fewer and shorter outbreaks after switching to a toothpaste without sodium lauryl sulfate (SLS), the foaming detergent in many conventional pastes. If your paste foams aggressively, switching to an SLS-free option is a cheap thing to test.
What is the fastest way to heal a canker sore?
Keep it clean and protected: warm salt-water rinses, a topical barrier gel or a pharmacist's mild corticosteroid paste for pain, and no acidic, salty, sharp, or very hot food on the spot. These steps shorten the episode and make it far less painful while it heals on its own.
Does a vitamin deficiency cause canker sores?
It can contribute. Low iron, folate, zinc, and B12 show up repeatedly in people with frequent ulcers, and correcting a genuine deficiency reduces outbreaks. If you get sores often, ask your doctor to check iron, B12, and folate rather than guessing with supplements.
Does chewing gum help or hurt canker sores?
It depends on timing. Chewing on an active sore can irritate it, so wait until it heals. Between outbreaks, sugar-free gum stimulates saliva, the mouth's natural buffer, and contains none of the SLS found in toothpaste. Gum is not a treatment for ulcers and will not heal one you already have.
A clean mouth without the detergent.
Minvelle is an SLS-free remineralizing gum with xylitol, Chios mastic, and nano-hydroxyapatite. No foaming detergent, just a low-irritant addition to a gentler routine.
Try Minvelle →Why saliva matters more than your toothpaste →
The fluid that buffers acid and helps mouth tissue recover.
Dry mouth: the silent driver of tooth decay →
When saliva drops, irritation and decay both climb.
Stress and your mouth: how cortisol weakens gums →
Why outbreaks cluster around your worst weeks.
- Herlofson BB, Barkvoll P. Sodium lauryl sulfate and recurrent aphthous ulcers, crossover trials. Acta Odontologica Scandinavica, 1994 and 1996.
- Scully C, Porter S. Recurrent aphthous stomatitis: clinical features and management. British Dental Journal, review.
- Brocklehurst P, et al. Systemic interventions for recurrent aphthous stomatitis. Cochrane Database of Systematic Reviews.
- Volkov I, et al. Vitamin B12 in the treatment of recurrent aphthous stomatitis. Journal of the American Board of Family Medicine, 2009.
- Belenguer-Guallar I, et al. Treatment of recurrent aphthous stomatitis: a literature review. Journal of Clinical and Experimental Dentistry, 2014.
- Edgar NR, Saleh D, Miller RA, The Journal of Clinical and Aesthetic Dermatology (Recurrent Aphthous Stomatitis: A Review), 2017
- Slebioda Z, Szponar E, Kowalska A, Archivum Immunologiae et Therapiae Experimentalis (Etiopathogenesis of Recurrent Aphthous Stomatitis and the Role of Immunologic Aspects), 2014
- Volkov I et al., Journal of the American Board of Family Medicine (Effectiveness of Vitamin B12 in Treating Recurrent Aphthous Stomatitis: A Randomized, Double-Blind, Placebo-Controlled Trial), 2009
- IQWiG / InformedHealth.org, NCBI Bookshelf (Overview: Canker sores / mouth ulcers), 2019
- Recurrent Aphthous Stomatitis - StatPearls, NCBI Bookshelf (prevalence, types, T-cell/TNF-alpha mechanism)
- Slebioda et al. Etiopathogenesis of Recurrent Aphthous Stomatitis and the Role of Immunologic Aspects: Literature Review, Archivum Immunologiae et Therapiae Experimentalis, PMC
- Evaluation of cytokine expressions in patients with recurrent aphthous stomatitis: systematic review and meta-analysis, PLoS One 2024, PMC
- Efficacy of Topical Intervention for Recurrent Aphthous Stomatitis: A Network Meta-Analysis, Medicina 2022, PMC
- Guideline for the Diagnosis and Treatment of Recurrent Aphthous Stomatitis for Dental Practitioners, J Int Oral Health, PMC
- Volkov et al. Effectiveness of Vitamin B12 in Treating Recurrent Aphthous Stomatitis: a Randomized, Double-Blind, Placebo-Controlled Trial, J Am Board Fam Med 2009
- Presence and distribution of dental enamel defects, recurrent aphthous lesions and dental caries in children with celiac disease, Pak J Med Sci, PMC
- Assessment of the prevalence of Behcet's disease in recurrent aphthous ulceration worldwide: a systematic review, Med Oral Patol Oral Cir Bucal 2025, PMC
- Genetic and immunologic findings in children with recurrent aphthous stomatitis with systemic inflammation, Pediatric Rheumatology Online Journal, PMC
Max, Founder of Minvelle. Reads dental research daily, not a medical professional. Every Minvelle post is fact-checked against primary sources, no LLM-generated content goes live unedited. More on how this brand started.
Last reviewed: June 2, 2026 by Max, Founder of Minvelle.