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Health condition · Clinically reviewed

Canker sores, triggers, topical steroids and the 3-week red flag.

Recurrent aphthous ulcers are common, painful and mostly benign. But a mouth ulcer that lingers over 3 weeks deserves a same-week review.

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Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a registered UK clinician before publication.

  • 02

    Sourced from guidance

    Checked against NICE CKS, BAOM and peer-reviewed oral medicine sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK guidance on recurrent aphthous stomatitis, topical steroids and 2WW oral cancer pathways.

Key facts

Canker sores at a glance.

The essentials, in plain English - what they are, how they differ from cold sores, and when to worry.

  • What it is

    Recurrent aphthous stomatitis (RAS) - round, painful mouth ulcers on non-keratinised mucosa. Not the same as a cold sore.

  • How common

    Very common - affects more than 20% of UK adults at some point, often starting in childhood or adolescence.

  • Three types

    Minor (~80%, under 10mm, heal in 7 to 14 days), major (~10%, over 10mm, scar) and herpetiform (~10%, pinpoint clusters).

  • Not a cold sore

    Cold sores are HSV, sit on the lip and outside the mouth, and start with tingling. Aphthous ulcers sit inside the mouth.

  • Triggers

    Trauma, stress, hormones, SLS toothpaste, nutrient deficiency (iron, B12, folate, zinc) and food sensitivities are the usual suspects.

  • When to worry

    Any ulcer lasting more than 3 weeks needs a same-week dental or GP review under the 2WW oral cancer pathway.

Why this guide matters

Three things to remember about mouth ulcers.

Canker sores are common and mostly self-limiting - but a small number of ulcers hide something more serious.

  • It is not a cold sore

    Aphthous ulcers sit inside the mouth on soft tissue and are not contagious. Cold sores are HSV and live on the lip.

  • Triggers usually explain flares

    SLS toothpaste, dental trauma, stress, hormones and nutrient deficiency drive most recurrent cases and are all addressable.

  • Three weeks is the red-flag rule

    Any single ulcer lasting more than 3 weeks - or a lump, red or white patch alongside it - needs an urgent 2WW oral cancer review.

How the diagnosis is made

From first ulcer to a clear plan.

The steps a UK GP, dentist or oral medicine specialist will normally follow, in order - so you know what to expect and why.

  1. 01

    Assessing

    History and pattern

    Frequency, duration, size, site, family history, medications and any systemic features - the pattern usually points to the diagnosis.

  2. 02

    Assessing

    Examination of the mouth

    A structured look at every surface - lips, buccal mucosa, tongue, floor of mouth, palate - plus a neck examination for lymph nodes.

  3. 03

    Assessing

    Systemic review

    Bowel symptoms, joint pain, eye inflammation, genital ulcers and skin rashes point to conditions such as coeliac, IBD or Behcet.

  4. 04

    Confirming

    Bloods for deficiency

    FBC, ferritin, B12, folate, zinc, coeliac serology (anti-tTG with total IgA), CRP and - where relevant - HIV testing.

  5. 05

    Confirming

    Biopsy if atypical

    A tissue biopsy for anything unusual - solitary, indurated, non-healing, involving attached gingiva, palate or floor of mouth.

  6. 06

    Escalating

    Specialist referral

    Oral medicine for refractory disease, gastroenterology if IBD is suspected, and rheumatology or dermatology if Behcet fits.

  7. 07

    Escalating

    2WW oral cancer pathway

    Any ulcer over 3 weeks, an unexplained neck lump or a persistent red or white patch triggers an urgent suspected-cancer referral.

Typical timeline: most cases resolve in one to two weeks; refractory patterns settle within a few specialist reviews.

Symptoms

What a canker sore actually looks like.

The classic look and feel of aphthous ulceration - and the features that mean it is time to escalate.

  • Round or oval ulcers

    Well-defined round or oval ulcers with a grey-white pseudomembrane and an inflamed red halo.

  • Prodromal tingling

    A burning or tingling patch 24 to 48 hours before the ulcer appears - a familiar warning sign in recurrent cases.

  • Pain on eating

    Sharp pain triggered by acidic, spicy or crunchy foods, brushing teeth and sometimes swallowing.

  • Minor aphthae

    Under 10mm, on lip, cheek or tongue - the commonest type, healing in 7 to 14 days without scarring.

  • Major aphthae

    Over 10mm, deeper, often on the soft palate or tonsillar pillars - can take 6 weeks to heal and leave scars.

  • Herpetiform ulcers

    Multiple pinpoint ulcers that coalesce into larger irregular lesions - despite the name, they are not caused by HSV.

  • Site inside the mouth

    Non-keratinised mucosa - inside of lips, cheek, under-tongue and soft palate. Attached gingiva and hard palate are unusual.

  • Red flag - over 3 weeks

    Any single ulcer that does not heal within 3 weeks needs same-week review under the 2WW oral cancer pathway.

Treatment

How canker sores are treated in the UK.

Trigger avoidance and simple topicals settle most flares. Steroids, tetracycline rinses and specialist systemic therapy handle the rest.

  • Trigger avoidance

    Switch to an SLS-free toothpaste (Sensodyne, Colgate SLS-free), soft brush, and cut back on acidic and abrasive foods during flares.

  • Nutritional repletion

    Correct iron, B12, folate or zinc deficiency where identified - often reduces flare frequency within weeks.

  • Chlorhexidine mouthwash

    A twice-daily antiseptic rinse that reduces secondary infection and helps ulcers settle faster.

  • Benzydamine (Difflam)

    A topical anti-inflammatory spray or rinse - takes the edge off pain and is well suited to eating and drinking.

  • Topical corticosteroids

    Hydrocortisone 2.5mg pellets, betamethasone soluble tablet as a mouthwash, or clobetasol ointment for severe or refractory cases.

  • Topical tetracycline rinse

    Doxycycline 100mg dissolved in water and swished for a few minutes - especially useful for herpetiform ulcers.

  • Oral systemic therapy

    Colchicine, pentoxifylline, dapsone, prednisolone or apremilast under specialist supervision for severe or Behcet-related disease.

  • Treat the underlying cause

    A gluten-free diet for coeliac disease, IBD-directed treatment, HIV care - address the driver and the ulcers often follow.

What this guide is based on

The sources behind every claim on this page.

UK national guidance and specialist society standards, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your GP, dentist or oral medicine specialist knows your history and can tell you which parts apply to you. If in doubt, get seen.

  • NICE CKS. Aphthous ulcer.

  • British and Irish Society for Oral Medicine (BSOM). Guidance on recurrent aphthous stomatitis.

  • NICE NG12. Suspected cancer: recognition and referral (oral cavity).

  • Scully C, Porter S. Oral mucosal disease: recurrent aphthous stomatitis. Br J Oral Maxillofac Surg.

Red flags

When a mouth ulcer needs urgent attention.

Most canker sores settle in a week or two. These are the features that mean it is time for a same-week review.

  • Ulcer lasting over 3 weeks

    Any single ulcer that has not healed in 3 weeks warrants a same-week review and consideration of the 2WW oral cancer pathway.

  • Ulcer on the tongue or floor of mouth

    These sites carry a higher risk for oral cancer, particularly with a history of smoking, alcohol use or HPV exposure.

  • Indurated, hard or fixed lesion

    A lesion with firm underlying tissue, an irregular edge or a mass beneath it needs urgent specialist assessment.

  • Persistent red or white patch

    Erythroplakia or leukoplakia sitting alongside an ulcer needs urgent oral surgery review.

  • Unexplained neck lymph node

    A hard or growing neck lump alongside a mouth ulcer is a red flag for head and neck malignancy.

  • Genital ulcers and eye inflammation

    Recurrent mouth ulcers plus genital ulcers, uveitis or joint pain point to Behcet disease - refer to rheumatology.

  • Bowel symptoms or weight loss

    Diarrhoea, blood in stool or unexplained weight loss alongside ulcers raises suspicion of IBD or coeliac disease.

  • Ulcers plus fever and cyclical pattern

    A regular monthly fever with mouth ulcers can indicate cyclic neutropenia or PFAPA and needs a full blood count.

  • Sudden severe ulceration on medication

    Nicorandil, methotrexate, NSAIDs or immunotherapy can cause severe mouth ulceration - review the drug urgently.

Living with it

A common condition, with a clear ladder.

Four things that make the biggest difference day to day - the right toothpaste, a gentler diet during flares, sleep and stress, and knowing when to escalate.

A quiet reminder

Small habits, kept up, beat heroic weeks.

Trigger avoidance and a topical steroid used at the first tingle usually shorten flares more than any late intervention.

  1. 01 Toothpaste

    Switch to SLS-free

    Sodium lauryl sulphate is a common trigger. An SLS-free toothpaste for 3 months is one of the highest-value changes you can make.

  2. 02 Diet

    Ease off the acid during flares

    Tomatoes, citrus, vinegar and fizzy drinks all sting. Room-temperature soft foods let ulcers settle faster.

  3. 03 Stress

    Protect sleep and stress

    Poor sleep and high stress reliably trigger flares. Simple sleep hygiene and short daily wind-down routines pay off.

  4. 04 Escalate

    See someone if it drags on

    Any single ulcer over 3 weeks or a big change in your usual pattern deserves a dentist or GP appointment this week.

Frequently asked

Everything we get asked about canker sores.

Quick answers on triggers, cold sore vs canker sore, and when to worry.

  • What is a canker sore?

    A canker sore is a recurrent aphthous ulcer - a painful round or oval ulcer inside the mouth on non-keratinised mucosa. It is different from a cold sore, which is caused by herpes simplex virus and sits on the lip and skin around the mouth.

  • What causes canker sores?

    The cause is multifactorial - genetics, immune activity and triggers such as trauma, stress, hormones, sodium lauryl sulphate toothpaste, and deficiencies of iron, B12, folate or zinc. In some people they are linked to coeliac disease, IBD, HIV or Behcet disease.

  • How is a canker sore different from a cold sore?

    Cold sores are caused by herpes simplex virus, start with tingling, appear on the lip or the skin around the mouth and crust over. Canker sores appear inside the mouth on soft tissue and are not contagious.

  • When should I worry about a mouth ulcer?

    Any single ulcer that has not healed within 3 weeks, an ulcer on the tongue or floor of mouth, an indurated or bleeding lesion, or an ulcer with a neck lump needs same-week review under the 2WW oral cancer pathway.

  • What is the best treatment for a canker sore?

    For most people - trigger avoidance, an SLS-free toothpaste, chlorhexidine mouthwash and benzydamine spray settle a flare. For severe or frequent ulcers a topical corticosteroid such as hydrocortisone pellets or a betamethasone mouthwash is very effective.

  • Can diet or vitamin deficiency cause mouth ulcers?

    Yes - low iron, B12, folate or zinc all increase ulcer frequency, and undiagnosed coeliac disease is a well-recognised cause. A GP can arrange the right blood tests, including coeliac serology.

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