Canker sore (aphtha) — how it looks, healing time and treatment (2026)

Q: Does vitamin B12 help canker sores?

Yes, in people with deficiency. Studies show clear reduction in recurrences with 1000 µg daily supplementation for 6 months — but test your levels first.

A small white-yellow spot inside your mouth that stings with every sip of coffee and every word. You know that feeling. Canker sores (aphthae) are one of the most common oral lesions — affecting up to 20% of adults — and in most cases they heal on their own in 7–14 days. This guide explains what a typical canker sore looks like, how it differs from cold sores, what actually speeds healing and when to stop waiting.

⚠️ Note: this article is information, not a diagnosis. Any ulcer that doesn't heal within 3 weeks, grows or bleeds — needs medical evaluation (dentist, oral surgeon or GP).

What a canker sore looks like

A classic canker sore (aphtha minor) is:

round or oval lesion, 2–8 mm in diameter,
white or white-yellow base (fibrin, not pus),
clear red inflammatory ring around it,
painful — especially with sour, salty, spicy food, talking and brushing,
appears on movable mucosa: inside of lips and cheeks, sides and underside of the tongue, floor of the mouth, soft palate.

Much rarer: aphtha major (>1 cm, deeper, heals in 2–6 weeks, may leave a scar) and herpetiform aphthae (tiny, numerous, 1–3 mm, in clusters).

💡 Important: canker sores are not contagious. Not a virus, not bacteria — it's an inflammatory reaction. You can't pass it via a kiss, cutlery or a glass.

Canker sore vs cold sore, thrush and other lesions

Lesion	Where	Look	Contagious?
Canker sore	inside the mouth (movable mucosa)	white-yellow ulcer with red ring	no
Cold sore (HSV-1)	usually on the lip / skin border	vesicles → crust, tingling at onset	yes
Oral thrush	tongue, palate, cheeks	white plaques that wipe off (leaves redness)	usually no
Leukoplakia	gum, cheek, tongue	white patch that cannot be wiped off	no, but needs evaluation
Oral cancer	sides of tongue, floor of mouth	ulcer not healing >3 weeks, hard borders	no

Vesicles on the lip = probably a cold sore, not a canker sore. A white patch you can't wipe off — don't self-diagnose, book a visit.

Why canker sores happen

The exact mechanism isn't fully understood, but common triggers:

micro-trauma — biting cheek, sharp tooth or brace edge, hard bread crust, aggressive brushing,
stress and sleep deprivation,
deficiencies: vitamin B12, folate, iron, zinc,
hormonal changes (menstruation, pregnancy),
SLS toothpaste (sodium lauryl sulfate) — worsens recurrences in some people,
acidic / spicy / salty food — pineapple, lemon, tomato, nuts, chocolate,
some medications (NSAIDs, beta-blockers, chemo),
systemic conditions: celiac disease, Crohn's, ulcerative colitis, HIV, Behçet's disease.

If canker sores appear monthly or more often, get CBC, ferritin, B12, folate and talk to your GP about celiac / IBD workup.

How long does it take to heal

Minor aphtha — the most common — 7–14 days, no scar.
Major aphtha — 2–6 weeks, may scar.
Herpetiform — up to 2 weeks, usually in clusters.

Sharp pain usually eases after 3–5 days as the ulcer base clears. If it isn't healing after 14 days and is still there at 21 days — get it checked.

What actually helps

Nothing shortens healing to 1 day. But you can significantly reduce pain and shorten healing by several days.

1. Topical anesthetics and barriers

lidocaine / benzocaine gels — numb before eating,
protective gels/pastes (with hyaluronic acid, carboxymethylcellulose) — form a shield over the sore,
hyaluronic acid preparations — speed epithelial regeneration.

2. Antiseptics and anti-inflammatories

chlorhexidine 0.12–0.2% mouthwash — twice daily for 5–7 days (may stain teeth long-term),
benzydamine gels/sprays — analgesic and anti-inflammatory,
low-dose topical steroid pastes (e.g. triamcinolone in adhesive paste) — by prescription, for recurrent/major aphthae.

3. Supportive

warm salt water rinse (1 tsp per glass) — soothes and cleans,
baking soda rinse (1 tsp per glass) — neutralizes acidic environment,
avoid acidic, spicy, hard, hot food and drink for a few days,
switch to SLS-free toothpaste if you get recurrences.

⚠️ Don't do this: don't apply alcohol or iodine, don't cauterize, don't pierce it. That's a chemical burn and delays healing.

Canker sores in children

In kids these are usually minor aphthae — painful but not dangerous. Similar treatment, but:

no chlorhexidine in children <6 without consultation,
lidocaine gels — cautiously in small children (aspiration risk); use pediatric formulations,
salt / soda rinse and paracetamol/ibuprofen in pediatric doses — safest start,
watch hydration — a child with a canker sore may refuse food and drink; offer cool, smooth foods (yogurt, jelly, purée).

If a child develops fever, multiple vesicles, gum swelling simultaneously — that's usually primary herpetic gingivostomatitis (HSV), not aphthae. See a pediatrician urgently.

When to stop waiting — red flags

Book a dental or GP visit if:

the ulcer doesn't heal within 3 weeks,
the lesion grows, hardens or bleeds,
you have fever or enlarged lymph nodes,
you get >3 canker sores per year, regularly recurring,
you also have chronic abdominal pain, diarrhea, weight loss (suspect celiac/IBD),
you smoke / drink alcohol and a lesion appears on the side of the tongue or floor of the mouth.

That last one matters: early oral cancer can look like a non-healing ulcer. Caught early it's curable — that's why any "canker sore" lasting a month deserves evaluation.

Check your photos in about a minute Not sure whether it's a canker sore, cold sore or something else? Take 5 phone photos in the free preDenti AI scan — get preliminary information and a hint on whether to see a dentist sooner.

How to prevent recurrences

Most effective in practice (and most often skipped):

SLS-free toothpaste (check the label — "sodium lauryl sulfate"),
soft brush, gentle brushing,
B12, folate, iron supplementation — after blood tests,
keep a recurrence diary for 2–3 months (food, sleep, stress, cycle) — patterns will emerge,
sharp edges of teeth/fillings/braces — ask your dentist to smooth them,
sleep hygiene and stress reduction — surprisingly effective.

FAQ

Is a canker sore contagious? No. Canker sores aren't caused by viruses or bacteria and don't spread through kisses, cutlery or glasses. It's a mucosal inflammatory reaction.

How long does it take to heal? A typical minor canker sore heals in 7–14 days. Major aphthae take 2–6 weeks. If it doesn't heal within 3 weeks, see a doctor.

Can a canker sore be cancer? A true canker sore — no. But a non-healing ulcer lasting more than 3 weeks, hardening or bleeding, can be early oral cancer and needs urgent evaluation.

Does chlorhexidine heal canker sores? It doesn't treat the cause but reduces inflammation and secondary infection risk. Shortens healing by a few days. Use max 5–7 days in a row.

Why do my canker sores keep coming back? Most common triggers: stress, deficiencies (B12, iron, folate), SLS toothpaste, micro-trauma, sometimes celiac or IBD. Get a CBC + ferritin + B12.

Can I kiss with a canker sore? Yes, they aren't contagious. May be physically uncomfortable.

When does a child's canker sore need a doctor? When it lasts >2 weeks, the child refuses to drink (dehydration risk), fever with multiple vesicles appears, or the child is <2 years old.

Does vitamin B12 help canker sores? Yes, in people with deficiency. Studies show clear reduction in recurrences with 1000 µg daily supplementation for 6 months — but test your levels first.

A canker sore usually looks scarier than it is — but a non-healing lesion is a signal you shouldn't ignore. If in doubt, check your photos in about a minute with preDenti AI.