I have a mouth ulcer that won't heal — when should I be worried?

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A mouth ulcer is one of the most common reasons people search online for medical information. The overwhelming majority of mouth ulcers are benign and self-limiting. A small minority are clinically significant — and the difference matters.

The simplest threshold: a mouth ulcer that has not healed after two weeks should be examined by a registered professional. Not three months, not "give it a bit longer". Two weeks.

This article is general educational content. The specific cause of any persistent ulcer in your mouth can only be established by a clinical examination by a registered specialist. If you have a persistent ulcer, please book a consultation; do not rely on this or any article for diagnosis.

How long should a mouth ulcer take to heal?

A typical traumatic or aphthous mouth ulcer heals within 10 to 14 days, passing through a predictable sequence of inflammation, granulation, and re-epithelialisation. An ulcer that has not healed in that window is no longer behaving like a self-limiting injury. Either there is an ongoing cause (something is still irritating it) or the lesion is not what you initially assumed.

The NICE referral guidance on suspected cancer uses three weeks as the threshold for routine referral of an unexplained oral ulceration.² We use two weeks for the threshold at which an examination is appropriate, recognising that an examination at two weeks often results in reassurance or simple management; if the lesion persists past three weeks, urgency increases.

The common causes of persistent ulcers

Mechanical trauma

A common cause. Repeated trauma from:

• A sharp tooth edge or broken restoration.
• An ill-fitting denture rubbing on the same spot.
• A new orthodontic appliance or brace.
• Habitual cheek-biting or lip-biting.

These ulcers heal within days once the cause is removed. They typically occur exactly at the point of irritation and have a predictable shape that matches the source.

Recurrent aphthous stomatitis (canker sores)

The most common chronic ulcerative condition of the mouth. Aphthous ulcers are round, well-defined, with a yellow or grey centre and a red border. They are painful, particularly when eating or drinking acidic foods.¹

Three sub-types:

• Minor aphthous ulcers — small (<10 mm), heal in 7–14 days without scarring. The most common.
• Major aphthous ulcers — larger (>10 mm), deeper, can take weeks to heal, sometimes leave scars.
• Herpetiform ulcers — multiple very small ulcers in clusters.

Recurrent aphthous ulceration affects an estimated 5–25% of the population at some point (Scully & Porter, 2008).¹ The exact cause is unknown; triggers include stress, hormonal change, nutritional deficiency, trauma, and some food sensitivities. Most cases are managed with topical treatment for symptomatic flares; persistent or severe cases warrant specialist evaluation.

Viral ulcers

• Primary herpetic gingivostomatitis (first contact with herpes simplex virus) — usually a childhood illness with multiple ulcers, fever, and lymphadenopathy.
• Recurrent herpes labialis ("cold sores") — at the lip border, usually clearly recognisable.
• Coxsackie virus infections (hand-foot-and-mouth disease, herpangina) — sometimes presenting in adults.

Viral ulcers typically come with other features (fever, multiple lesions, prodromal symptoms) that distinguish them.

Medication-related ulceration

A growing category as drug use ages. Several medications can cause oral ulceration:

• Some chemotherapy drugs.
• Bisphosphonates (and other antiresorptive agents).
• Methotrexate.
• Some NSAIDs.
• Nicorandil (anti-anginal medication; characteristically causes painful ulcers).

A new medication started in the weeks before an ulcer appeared is a clue worth taking seriously.

Systemic causes

Less common but worth considering:

• Crohn's disease and ulcerative colitis — can present with oral ulceration.
• Behçet's disease — recurrent oral and genital ulceration plus other features.
• Coeliac disease — sometimes presents with recurrent oral ulcers.
• HIV infection — can produce a range of mucosal lesions including ulcers.
• Autoimmune blistering diseases — pemphigus, mucous membrane pemphigoid.
• Nutritional deficiency — iron, B12, folate, zinc deficiencies.

For persistent ulceration without obvious local cause, blood tests via the patient's GP are often part of the workup.

Is a mouth ulcer that won't heal always cancer?

No. The large majority of persistent mouth ulcers are not cancer — most are traumatic, aphthous, viral, or medication-related. The reason a non-healing ulcer is taken seriously is that oral cancer cannot be ruled out by appearance alone, and it is the diagnosis where early detection most affects the outcome. The clinical features that increase the suspicion of an oral squamous cell carcinoma include:

• Persistence beyond three weeks without obvious cause.
• Site — the lateral or ventral (underside) surface of the tongue, the floor of the mouth, and the soft palate are higher-risk sites.
• Edges that are raised, rolled, or indurated (firm to touch).
• Painlessness — oral cancers are often less painful than benign ulcers of equivalent size.
• Bleeding that is easily provoked or spontaneous.
• A lump in the floor of the mouth or in the neck (a lymph node) accompanying the ulcer.
• Risk-factor history — smoking (including former smoking), heavy alcohol use, betel-nut/areca use, prior head-and-neck cancer.

Cancer of the lip and oral cavity accounted for an estimated 377,713 new cases worldwide in 2020 (GLOBOCAN), with the burden concentrated in tobacco- and alcohol-using populations.³ In South Africa, tobacco (smoked and smokeless) and alcohol are the leading modifiable risk factors. Early detection materially changes outcomes: oral-cavity cancer found at a localised stage has a 5-year relative survival of about 89%, falling to about 38% once it has spread to distant sites (NCI SEER).⁴

The honest framing: the prior probability that any specific persistent ulcer is cancer is low. The reason for the two-week / three-week threshold for examination is not that cancer is the likely diagnosis — it is that cancer is the diagnosis where early detection most affects outcome, and it cannot be ruled out without a clinical look.

What to do if you have an ulcer

For an ulcer less than two weeks old:

• Avoid spicy, acidic, salty, or sharp foods.
• Use a soft-bristled toothbrush.
• Switch to an SLS-free toothpaste during the episode.
• Topical anaesthetic gel (over the counter) for symptomatic relief.
• A salt-water mouthwash several times a day.
• If the cause is obvious (a sharp tooth, a denture problem), arrange to have it addressed.

At two weeks if not healed:

• Book an appointment with your dentist or an oral medicine specialist.
• Take a photograph of the ulcer for record and comparison.
• Make a note of when it appeared, what you have tried, and any associated symptoms.

At three weeks:

• If you have not already seen someone, do so as a priority.
• Mention any of the higher-risk features listed above.

Practical: what NOT to do

• Do not ignore an ulcer because it does not hurt. Lack of pain is not reassurance for a persistent oral ulcer.
• Do not apply over-the-counter topical antibiotics or antifungals indefinitely without diagnosis.
• Do not wait six weeks "to see what happens".
• Do not rely on this article (or any internet content) for diagnosis. Persistent oral lesions need a clinical examination.

This article is general educational content. The specific cause of any persistent oral ulcer can only be established by clinical examination, sometimes supplemented by biopsy.