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Medically reviewed guide · Last updated: 1 August 2026 · Reading time: approx. 8 min
Mouth Ulcers: What Triggers Those Sore Spots — and What Really Helps
At a glance
Mouth ulcers — clinically aphthous ulcers — are small, inflamed breaks in the lining of the mouthroundish, with a whitish-yellow coating and a red rim. They sting above all when you eat and speak, but they are not contagious.
Three formsminor ulcers (by far the most common, less than a centimetre across, healing within one to two weeks without a scar), major ulcers (larger and deeper, taking up to six weeks and capable of leaving scars) and herpetiform ulcers (many pinhead-sized lesions at the same time).
The exact cause is unclear on current knowledge.Known triggers are stress, small injuries to the lining of the mouth, certain toothpaste ingredients and nutrient deficiencies (iron, vitamin B12, folate).
Medicines can be the trigger too— NSAID painkillers, for instance, the angina medicine nicorandil, or methotrexate.
Treatment eases things, but time is what healsnumbing gels and protective pastes make the days more bearable, yet they speed up healing only marginally at best.
Get it checked without delay ifa sore spot has not healed after two to three weeks, mouth ulcers appear together with ulcers in the genital area or with inflammation of the eye, or you feel markedly unwell with a fever.
A mouth ulcer, cold sore or something else? The comparison
Not every sore spot in the mouth is a mouth ulcer. Where it sits is the most important first clue: mouth ulcers sit inside, on the mobile lining, while a cold sore is almost always outside, on the edge of the lip.
Cause
Typical appearance
Typical location
Healing
First step
Mouth ulcer (minor)
Roundish, with a yellowish coating and a red rim, very sensitive to touch
Inside: the lining of the cheek, the inside of the lip, the edge of the tongue
1–2 weeks, without a scar
Eat bland food, use a numbing gel, look for triggers
Cold sore
Grouped blisters that burst and crust over, with tingling beforehand
Outside, on the edge of the lip, rarely on the hard palate
1–2 weeks
Antiviral cream as early as possible, hygiene (it is contagious!)
Mechanical injury
Irregular, matching the point of irritation (a bite, a sharp edge of a tooth, a denture, a brace)
Exactly where something rubbed or pressed
A few days, once the cause is gone
Remove the cause, have the edge smoothed at a dental practice if needed
Oral thrush (candida)
Whitish coating that wipes off, with reddened, sore lining underneath
Over large areas: tongue, cheeks, palate
With antifungal treatment
Get it assessed medically, look for the trigger
Suspicious lesion
Irregular, often hardened, frequently not very painful, changing or getting larger
Possible anywhere, often the edge of the tongue, the floor of the mouth, the lower lip
Does not heal within 2–3 weeks
Have it assessed by a dentist or an ENT practice
Table scrolls to the right
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Minor ulcers make up by far the largest share. They are smaller than a centimetre, flat, sit singly or in pairs on the mobile lining and heal completely within one to two weeks — without a scar. Unpleasant as they are, they are harmless.
Major ulcers are larger than a centimetre, reach deeper into the tissue and hurt correspondingly more. Healing can take several weeks, up to six, and scars can be left behind. Major ulcers should be followed up by a doctor or a dentist — not least in order to rule other causes out reliably.
Herpetiform ulcers are the rarest form: dozens of pinhead-sized lesions appear at the same time and can merge into larger sore areas. The name is misleading — they have nothing to do with herpes viruses, they simply look similar to herpes blisters. The difference from true herpes: mouth ulcers sit inside the mouth and are not contagious, whereas a cold sore sits outside and very much is.
Why mouth ulcers develop: an honest answer
The short and unsatisfying truth: on current knowledge the underlying cause has not been established. An excessive local immune reaction of the lining is suspected, with several factors acting together. A family clustering is well described — if your parents are prone to mouth ulcers, your own likelihood is higher. What can be said with confidence: mouth ulcers are not an infection, you cannot pass them on to anyone, and poor hygiene is not the cause.
Triggers: stress, injuries and toothpaste
Even though the underlying cause is an open question, some triggers are well documented or at least plausible:
Small injuries — a bite to the cheek, a hard piece of crispbread, the toothbrush, orthodontic appliances. Mouth ulcers develop by preference where the lining has been irritated.
Stress and exhaustion — many people report flares during demanding phases. The study data on this are limited, but the pattern is strikingly common.
Sodium lauryl sulfate (SLS) in toothpaste — the foaming agent is suspected of irritating the lining of the mouth. The evidence is not clear-cut; but switching to an SLS-free toothpaste for a limited period is a low-risk experiment that can be worth it if you get mouth ulcers often.
Certain foods — very acidic, spicy, hard or salty foods make existing ulcers worse almost every time; whether they trigger new ones varies a great deal from person to person.
Hormonal fluctuations and stopping smoking — paradoxically, some people report temporarily more mouth ulcers after giving up smoking. That is no reason to keep smoking, but it is useful to know so that you read the connection correctly.
Nutrient deficiencies: iron, B12 and folate
Some of the people who get recurrent mouth ulcers turn out to be short of iron, vitamin B12 or folate. That is why a blood test belongs in the work-up when they keep coming back — whereas taking supplements off your own bat is not sensible, because an undetected deficiency can have a cause that needs investigating. Typical accompanying signs of iron deficiency are fatigue, pallor and brittle nails; there is more on this in the article on iron-deficiency anaemia.
When mouth ulcers keep coming back: think about an underlying condition
The odd mouth ulcer a few times a year is no cause for concern. But if they come very often, in flares, or together with bowel symptoms, it is worth looking specifically: coeliac disease (gluten intolerance) and inflammatory bowel diseases such as Crohn's disease can also show themselves — sometimes first of all — in the lining of the mouth. Very rarely, Behçet's disease is behind it, an inflammatory condition of the blood vessels in which mouth ulcers occur together with ulcers in the genital area and inflammation of the eyes. A weakened immune system, for instance through HIV infection or immunosuppressant medicines, also makes severe and poorly healing ulcers more likely.
Self-observation: first clues at home
These checks do not replace a diagnosis — but they will help you make the conversation at your dental or GP practice more concrete.
The location check: does the spot sit inside, on the mobile lining (which points to a mouth ulcer), or outside on the edge of the lip with blisters (which points to herpes)?
The timing check: note the date it appeared. A typical minor ulcer is clearly better after two weeks at the latest. Anything unchanged after two to three weeks needs to be checked.
The pattern check: if they keep recurring, keep a short diary — date, number, location, and what you ate, experienced or newly started taking in the days beforehand. Three documented flares say more than any memory.
The product and medication check: a new toothpaste, a new mouthwash, a new medicine in the past few weeks? This is one of the simplest and most frequently overlooked trails.
The accompanying-signs check: fatigue, pallor, weight loss, diarrhoea or abdominal pain on top of the ulcers? Then a full blood count, and possibly a bowel assessment, belong on the plan.
Warning signs: when not to wait
A sore spot that has not healed after two to three weeks — even if it does not hurt
Mouth ulcers together with ulcers in the genital area or with recurrent inflammation of the eyes
Very large (over a centimetre) or unusually deep lesions
Feeling markedly unwell, fever or swollen lymph nodes
Pain so severe that eating and drinking are barely possible — especially in children (risk of dehydration)
Frequent flares together with diarrhoea, abdominal pain or weight loss
Take spots that do not heal seriously A single spot in the mouth that has not healed after two to three weeks, that hardens or gets larger, is usually harmless — but in rare cases it can be a pre-cancerous change or a cancer of the mouth, particularly if you have smoked for many years or drink alcohol regularly. That is precisely why the two-to-three-week rule applies without exception: have it looked at by a dentist or an ENT practice. Caught early, changes like these can usually be treated well — waiting is the one wrong decision here.
The treatment pathway: step by step
Mouth ulcers heal on their own — treatment aims to ease the pain, protect the spot and make the time until it heals bearable.
Eat and drink gently. Soft, bland, rather cool food; a straw for spots on the inside of the lip. Acid, spice, alcohol and anything very hot reliably make the pain worse.
Numb it locally. Gels or sprays containing a local anaesthetic (lidocaine, for example) take the pain away for meals and for brushing your teeth. They work only briefly, but they can be used deliberately before eating — follow the instructions in the patient information leaflet.
Cover and protect. Hyaluronic acid gels and adhesive pastes lay a protective film over the spot. That eases the sensitivity to touch; a clear effect on how long healing takes is not reliably established.
Rinse with an antiseptic. Chlorhexidine mouthwashes can prevent additional bacterial colonisation and are also used against bad breath. Important: use them for a limited time only — with longer use you risk staining of the teeth and a temporary taste disorder.
For frequent recurrences or major ulcers: get it assessed medically. A full blood count with iron, vitamin B12 and folate makes sense, possibly coeliac serology as well, along with a review of your medication list. In severe cases the practice may consider prescription-only options, such as adhesive pastes containing a corticosteroid — that decision always rests with the practice treating you.
Home remedies, honestly assessed. Chamomile or sage tea rinses are mild and worth a try — but there is no solid evidence that they speed up healing. Some widely shared tips are advised against by specialists: salt or bicarbonate of soda applied directly to the ulcer stings badly and irritates it further, undiluted tea tree oil can damage the lining of the mouth, and hydrogen peroxide has no place in the mouth undiluted. As a rule of thumb: whatever stings badly on an open sore is not helping it.
The medication angle: when medicines are the trigger
Mouth ulcers and ulcer-like breaks in the lining can be triggered or made worse by medicines — which is why a look at your own medication list is always worth it, especially if the symptoms fit in time with something you have newly started.
NSAID painkillers such as ibuprofen, naproxen or diclofenac have been described as triggers of ulcer-like lesions. That is rare — but if you take NSAIDs regularly and repeatedly get mouth ulcers, it is worth raising the connection.
Nicorandil, a medicine for angina, is a well-known trigger of stubborn, often large ulcers of the lining — in the mouth, but on other linings too. They typically only heal once the medicine is changed under medical supervision.
Methotrexate can cause painful inflammation and breaks in the lining of the mouth as a side effect (mucositis). Sore spots while on methotrexate are a warning sign that belongs in front of your practice promptly — they can indicate too high an exposure to the drug. The folate that is usually prescribed alongside it reduces this risk; so take it just as reliably as the methotrexate itself.
Chemotherapy attacks fast-growing cells — including those of the lining of the mouth. Mucositis is a common and well-known accompanying effect here, actively looked after by the treatment team; sore spots should be reported to them early.
Suspect it, yes — stop it, no Never stop a prescribed medicine on your own, even if you suspect it of being the trigger — with medicines such as methotrexate or nicorandil that can have serious consequences. The right approach: document what you have observed and discuss it at your practice. How to recognise and report side effects systematically is set out in the guide side effects of medicines; what matters when stopping something under medical supervision is covered under stopping medicines.
Your medicine and your mouth ulcers — coincidence or pattern?
What you take and how you feel, side by side over time: that is how a suspicion becomes something you can discuss.
Brush gently but thoroughly — a soft brush and a calm technique avoid micro-injuries. Good oral hygiene also protects you from bleeding gums and periodontitis — even if it does not reliably prevent mouth ulcers.
Try an SLS-free toothpaste — if you get mouth ulcers often, try it for two to three months and compare how often the flares come. The evidence is limited, the experiment risk-free.
Have sharp edges dealt with — protruding edges of fillings, dentures that press or brackets irritate the lining continuously. A short appointment at a dental practice often sorts this out in minutes.
Know your own triggers — keep a diary if the flares keep recurring. Frequently named candidates are hard bread crusts, nuts, citrus fruit, tomatoes and very spicy dishes — which of them affect you is something only your own observation will show.
Get deficiencies checked instead of guessing — if they keep recurring, have iron, B12 and folate measured before you start taking supplements. A proven deficiency is treated specifically; a suspected one is just expensive.
Take stressful phases seriously — sleep, breaks and exercise are no guarantee against mouth ulcers, but many people see their flares exactly when all of that falls by the wayside.
Spot the flares instead of trying to remember them
Symptoms, triggers and medicines documented in one place — ready for your next appointment.
No. Mouth ulcers are not caused by viruses or bacteria and cannot be passed on by kissing or by sharing crockery. That sets them apart from cold sores, which are highly contagious and sit on the outside edge of the lip.
The most reliable option is a numbing gel containing a local anaesthetic, applied deliberately before eating. Protective pastes and hyaluronic acid gels reduce the sensitivity to touch over a longer stretch. Healing itself can barely be sped up — after one to two weeks a minor ulcer has cleared up by itself.
Recurrent mouth ulcers are common and often run in the family without any underlying condition. You should still have it looked into: iron, B12 or folate deficiency, coeliac disease or inflammatory bowel disease can present this way. Medicines such as NSAIDs, nicorandil or methotrexate are possible triggers too.
Possibly. The foaming agent sodium lauryl sulfate (SLS) is suspected of irritating the lining of the mouth — though the study data are not clear-cut. Trying an SLS-free toothpaste for two to three months is risk-free and worth a go if you get mouth ulcers often.
When a spot has not healed after two to three weeks, when it is unusually large or deep, when fever or a strong feeling of being unwell comes with it, or when mouth ulcers appear together with ulcers in the genital area or inflammation of the eyes. Very frequent flares need looking into as well — including a full blood count and a look at your medication list.
Chamomile or sage rinses are mild and can be pleasant as an addition, but their benefit is not established. Salt or bicarbonate of soda applied directly to the sore spot stings badly and irritates it further, and undiluted tea tree oil can damage the lining of the mouth. Rule of thumb: whatever stings badly on an open sore is not helping it.
Mouth ulcers sit inside the mouth on the mobile lining, are flat, coated breaks in the surface, and are not contagious. A cold sore is caused by a virus, sits almost always on the outside edge of the lip, starts with tingling and grouped blisters — and is contagious. If you are unsure, the location is the first thing to go by: inside points to a mouth ulcer, outside to herpes.
gesund.bund.de: mouth ulcers and inflammation of the lining of the mouth — German source. Accessed 2026.
German S2k guideline on the diagnosis and treatment options for aphthous and aphthous-like lesions of the oral and pharyngeal mucosa (DGMKG/DGZMK, AWMF register) — German source. Accessed 2026.
Summaries of product characteristics for the active ingredients mentioned (methotrexate, nicorandil, NSAIDs). Accessed 2026.
This article is for general information and does not replace medical advice, diagnosis or treatment. The self-observations described are a guide only and not a diagnosis. If you have sore spots in your mouth that have not healed after two to three weeks, mouth ulcers together with genital ulcers or inflammation of the eyes, or a high fever or a marked feeling of being unwell, please contact a dental practice, a GP practice or the out-of-hours medical service.
