Canker Sore Won't Heal After 2 Weeks — What's Normal and What Needs Attention

TL;DR

An ordinary canker sore usually heals in one to two weeks, so day 14 is the outer edge of normal, not an emergency. What matters is whether it's getting better. A sore that's smaller and less painful than it was a week ago is on its way out. A sore that looks the same as it did on day 4 is stuck. Large canker sores (wider than about 1 cm) normally take two to six weeks. The most common reason a sore won't heal is that something keeps re-injuring it: a sharp tooth, a broken filling, a brace wire, a habit of biting the spot. Some medications cause sores that won't heal until the drug is stopped. Any mouth ulcer that hasn't clearly improved after three weeks needs to be seen by a dentist, and UK cancer guidelines recommend an urgent referral at that point (NICE NG12). Most of these turn out to be harmless, but it's the point where cancer has to be ruled out, and catching it early makes an enormous difference.

What Normal Healing Looks Like

Ordinary (minor) canker sores are smaller than about 1 cm and usually heal in 7 to 14 days without scarring. The pain is usually worst in the first few days, then eases as the sore starts closing in from the edges.

Large (major) canker sores are wider than about 1 cm and deeper. They normally take two to six weeks, can be extremely painful, and sometimes leave a scar. See Giant Canker Sores.

Herpetiform canker sores are clusters of many tiny ulcers, which sometimes merge. They usually heal within a week or two, but new ones can keep appearing for longer. See Herpetiform Canker Sores.

So before you worry, figure out which kind you have. A 15 mm ulcer that's still there at day 14 is behaving normally. A 4 mm one that hasn't changed at all since day 4 is not.

Questions to ask at two weeks

• Is it smaller than it was at its worst?
• Does it hurt less than it did last week?
• Is the yellow-white center shrinking, and is the red rim fading?

If the answers are yes, it's healing, just slowly. Give it more time. If the answers are no, and it looks the same as it did in the first few days, it's stuck, and it's worth finding out why.

Why a Canker Sore Gets Stuck

Something keeps hitting it

This is the most common reason by far. A sore can't heal if it's re-injured several times a day. Common culprits:

• A sharp or chipped tooth edge, or a broken filling
• Braces, a wire end, or a retainer. See Canker Sores From Braces.
• A denture or partial that rubs
• Biting or chewing the spot, often without noticing, especially during stress or sleep
• Brushing hard right over the sore

A good test: if the sore sits right where something touches it, fixing that thing (having the tooth smoothed, the appliance adjusted, waxing the bracket) should let it heal within a week or two. If it does, that was the problem. See Why Canker Sores Keep Coming Back in the Same Spot.

It's a large canker sore

Major canker sores simply take longer, and over-the-counter products don't do much to speed them up. They usually need prescription treatment. Topical steroids help canker sores heal faster than placebo (systematic review, 2008 — PMID: 18588791), and a dentist can prescribe a stronger one for a large sore.

Low B12, iron, folate, or zinc

The lining of your mouth needs these to rebuild. If you're deficient, sores may heal slowly and come back often. If you haven't been tested, it's worth doing. See Why Doctors Dismiss Canker Sores for which tests to ask for.

Smoking

Smoking reduces blood flow to the lining of the mouth and slows healing in general. (Oddly, smokers tend to get fewer canker sores, but the sores they do get heal like any other wound, which is to say more slowly.)

Medications

Some drugs cause mouth ulcers that look like canker sores but won't heal as long as you keep taking them.

• Nicorandil, a heart medication used mainly in the UK and Europe, is a well-documented cause of large, painful, non-healing mouth ulcers. They typically heal within one to two months after the drug is stopped (Yamamoto et al., 2011 — PMID: 21872503).
• Methotrexate, used for rheumatoid arthritis, psoriasis, and Crohn's disease, can cause mouth ulcers, especially if folic acid isn't taken alongside it or kidney function changes. Mouth sores on methotrexate are worth reporting to your prescriber promptly, because they can be an early sign of toxicity.
• Bone-strengthening drugs (bisphosphonates and denosumab, used for osteoporosis and cancer) can rarely cause osteonecrosis of the jaw, where an area of jawbone dies and shows through the gum. It can look like a sore that won't heal, especially after a tooth extraction. Exposed bone that doesn't heal is the key sign (Ruggiero et al., 2022 — J Oral Maxillofac Surg).
• NSAIDs like ibuprofen and naproxen are linked to canker sores in some people. See What Causes Canker Sores.

Don't stop a prescribed medication on your own. Tell your prescriber about the sore, since some of these need a dose change or a different drug.

The Three-Week Rule

Any mouth ulcer that hasn't clearly improved after three weeks needs to be looked at by a dentist or doctor. The UK's national cancer guideline puts it this way: consider a suspected cancer referral, to be seen within two weeks, for people with unexplained mouth ulceration lasting more than three weeks (NICE NG12).

That's not because most of these sores are cancer. Most aren't. It's because by three weeks, a normal canker sore should be clearly on its way out, and if it isn't, the list of possible explanations gets longer and includes some that need treatment.

Don't wait for six or eight weeks hoping it will sort itself out. If you're at three weeks without improvement, book the appointment.

What Else a Non-Healing Sore Can Be

An injury from something you haven't noticed

Covered above. It's the most common explanation, and the easiest to fix.

Lichen planus

An immune condition that affects the lining of the mouth, often showing up as lacy white lines on the inside of the cheeks. The erosive form causes raw, red, painful areas that don't follow the usual canker sore cycle, often on both sides of the mouth or along the gums. It usually needs a biopsy to diagnose and is treated with topical steroids or other medications from an oral medicine specialist.

Mucous membrane pemphigoid and pemphigus

These are autoimmune blistering diseases. Blisters form on the lining of the mouth, break, and leave raw sores that keep coming. Pemphigoid often shows up as red, peeling, sore gums. Diagnosing them takes a biopsy along with a special test on the tissue (direct immunofluorescence) or blood tests that look for the antibodies involved.

Infections

• Syphilis. A primary syphilis sore (chancre) in the mouth is usually a single, firm, painless ulcer, and the mouth is the most common place for one outside the genitals. It heals on its own in about a month, but the infection doesn't go away. A blood test diagnoses it, and it's treated with antibiotics.
• Herpes in people with weakened immune systems can cause large, slow-healing sores rather than the usual small clusters.
• Rarer infections, like tuberculosis or deep fungal infections, can cause chronic ulcers in people with specific risk factors.

Cancer

This is the reason the three-week rule exists. Oral cancer can start as a sore that doesn't heal. It's much less common than any of the explanations above, but it's the one that can't be missed.

Features that make a sore more concerning:

• A hard, thick, or raised edge. Canker sores have soft, flat edges. This is the single most important sign.
• Little or no pain despite its size. Canker sores hurt. Early cancers often don't.
• Its location. The side of the tongue toward the back and the floor of the mouth (under the tongue) are the most common sites for mouth cancer. On the lip, cancer usually shows up on the lower lip in people with a lot of sun exposure.
• A red or white patch around or near the sore.
• Numbness, a lump in the neck, or a denture that suddenly doesn't fit.
• Risk factors: smoking or chewing tobacco, heavy drinking, betel quid, and age over 40. People without any risk factors can still get oral cancer, though.

Here's why it's worth the appointment. For cancers of the mouth and throat, about 88% of people whose cancer is found while it's still localized are alive five years later. Once it has spread to distant parts of the body, that drops to about 37% (SEER, National Cancer Institute). Only about a quarter of cases are caught at the localized stage. A dental visit to check a stubborn sore is how more of them get caught early.

What Happens at the Appointment

Knowing what to expect makes it easier to actually go.

The exam. The dentist will look at the sore, feel its edges and the surrounding tissue with a gloved finger, and check your neck for swollen lymph nodes. They'll ask how long it's been there, whether it hurts, and what you've tried.

Fixing the obvious. If there's a sharp tooth, rough filling, or rubbing appliance next to the sore, they'll usually smooth or adjust it and ask you to come back in a week or two to see if it's healing.

Treatment trial. For a sore that looks like a large canker sore, they may prescribe a steroid gel or rinse and recheck it.

Biopsy. If the sore still isn't healing, or it has features that need checking right away, they'll take a biopsy or refer you to someone who will. It sounds worse than it is:

• The area is numbed with a local anesthetic injection, the same as for a filling.
• For a small sore, the whole thing may be removed (an excisional biopsy). For a larger one, a small piece is taken from the edge (an incisional biopsy).
• It usually takes a few minutes. You may get a stitch or two.
• The area is sore for a few days afterward, roughly like a bad canker sore.
• Results usually come back in one to two weeks.

If an autoimmune condition like pemphigoid is suspected, a second small sample may be taken from nearby healthy-looking tissue for the special immunofluorescence test.

Most biopsies of stubborn mouth sores come back as something benign and treatable. The biopsy is how you find that out for certain instead of guessing.

What to Do Right Now

If it's improving at two weeks, even slowly: keep up the home care, avoid anything that hits it, and give it another week.

If it looks the same at two weeks as it did in the first few days: look for something that's rubbing it, and book a dental appointment. You don't need to wait for day 21.

If it has a hard edge, doesn't hurt, or is on the side of the tongue or floor of the mouth: call a dentist this week.

If it's been three weeks without clear improvement: book the appointment now.

In the meantime, protecting the sore from food and your teeth can make it more comfortable:

For a broader guide to when a mouth sore needs attention, see When to See a Doctor for a Canker Sore.