The short answer
A sore on the skin that hurts, itches, bleeds or crusts for more than 4 weeks should be seen by a GP. In the mouth the number is 3 weeks, and a dentist can start that conversation.
The NHS says to see a GP about a growth or area of skin that hurts, itches, bleeds, crusts or scabs for more than 4 weeks.
The NHS also lists a growth on the skin that is getting bigger, or has changed colour or texture, as a reason to book.
For the mouth, the NHS number is different: an ulcer lasting more than 3 weeks, or a red or white patch, should be seen by a GP or dentist.
NCI lists a sore on the lip or in the mouth that does not heal among lip and oral cavity cancer symptoms, along with a lump, a red or white patch, loose teeth and jaw swelling.
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The full explanation.
Two numbers, depending where the sore is
Skin and mouth have different clocks, and mixing them up is the commonest mistake here.
For skin, the NHS says to see a GP if you have a growth or area of skin that hurts, itches, bleeds, crusts or scabs for more than 4 weeks. It also lists a growth that is getting bigger or has changed colour or texture.
For the mouth, the number is 3 weeks. The NHS says to see a GP if you have a mouth ulcer that has lasted more than 3 weeks, or a red or white patch inside your mouth.
Both are non-urgent advice. That means book an appointment, not go to A&E.
The sore that reopens
Some sores never quite finish. They scab, look settled for a week, then break down in the same spot.
That pattern is worth describing in exactly those words when you book. It is different from a wound that is slowly shrinking, and it is what the NHS wording about bleeding, crusting and scabbing over four weeks is designed to catch.
Take a photo the first time it opens. If it reopens, you will have proof it is the same place rather than a new knock.
Where these sores usually appear
The NHS says the main symptom of non-melanoma skin cancer is a growth or unusual patch on the skin, and that it is most common in areas exposed to the sun: the head, face and ears, the neck and shoulders, the back, the hands and the lower legs.
It describes the range these can take. Some growths are raised and smooth with clear edges. Some are bumpier, rough or crusty. Some start as a flat discoloured patch, and colours can include purple, brown and red, which may look darker on brown or black skin.
That variety is the point. There is no single look to memorise.
Why skin fails to close
Cancer is not the usual reason a sore lingers.
Pressure keeps reopening skin over a heel, a hip or the base of the spine. Poor circulation starves a leg ulcer of what it needs to heal. Diabetes slows healing and can dull the pain that would normally make you shift position. Infection stalls a wound. Eczema, psoriasis and repeated scratching can keep a patch raw for months. Steroids and some other medicines thin the skin.
Most of these have treatments, which is another reason to have it looked at rather than covered up.
The mouth version
NCI lists a sore on the lip or in the mouth that does not heal among lip and oral cavity cancer symptoms. The rest of its list is a lump or thickening on the lips, gums or in the mouth, a white or red patch, bleeding, pain or numbness in the lip or mouth, a change in voice, loose teeth or dentures that no longer fit, trouble chewing, swallowing or moving the tongue or jaw, jaw swelling, and a sore throat or a feeling of something caught.
NCI adds that these cancers may cause no symptoms at all, and are sometimes found during a regular dental exam. A dentist is a reasonable first stop.
When a sore becomes urgent
Infection is the one thing here that changes within hours.
The NHS says to call 999 or go to A&E if an adult is breathing very fast, is confused or slurring words, has blue, pale or blotchy skin, has a very high or very low temperature or feels shivery, or has a rash that does not fade when pressed. Those are its sepsis signs.
Otherwise, book. Our page on preparing for appointments explains what to bring.
Sources
Words to know
Tap any term to see what it means.
Common questions
How long should a sore take to heal?
The NHS uses 4 weeks for skin. A growth or area that hurts, itches, bleeds, crusts or scabs for longer than that is a reason to see a GP. For a mouth ulcer the number is 3 weeks.
It keeps healing and then breaking open again. Does that count?
Yes, and it is worth saying in those words. The NHS advice covers skin that bleeds, crusts or scabs over that period, and a spot that repeatedly reopens in the same place fits that description.
Where do these sores usually turn up?
The NHS says non-melanoma skin cancer is most common on sun-exposed skin: the head, face and ears, the neck and shoulders, the back, the hands and the lower legs.
Questions to ask your doctor
Being prepared helps you get the most out of your appointments. Save or print these questions.
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Your next step
Write down timing, severity, changes, and questions before you talk with a clinician.
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Sources last checked: 2026-08-09 what this meansLast updated: 2026-08-09Next planned review: 2027-07-20
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Editorial review complete — This page completed Cancer Explained's editorial checks (sources, safety, plain language, duplication). It has not been reviewed by a physician or other healthcare professional.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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