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.
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 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.
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.