TL;DR
Canker sores are non-contagious, painful oral ulcers (aphthous ulcers) with no single known cause. About 20% of people get them regularly. There are three types — minor, major, and herpetiform — each with different healing times and severity. They are not cold sores (which are caused by HSV-1 and are contagious). Most resolve on their own in 1–2 weeks.
What Is a Canker Sore?
A canker sore — medically termed an aphthous ulcer or recurrent aphthous stomatitis (RAS) — is a shallow, painful ulcer that forms on the soft tissues inside the mouth: the cheeks, inner lips, tongue, soft palate, or at the base of the gums. Unlike cold sores, they do not appear on the outer lip, and they are not caused by a virus.
The word aphthous comes from the Greek aphtha, meaning "ulcer." The condition has been described in medical writing since ancient times, yet its exact cause still isn't fully understood.
How common is it? About 20% of people get recurring canker sores at some point in their lives, making it the most common condition affecting the lining of the mouth. It's reported more often in higher-income groups — but that's likely because those people are more likely to see a doctor and get diagnosed, not a real biological difference.
The Three Types of Canker Sores
Minor Aphthous Ulcers
The most common form, roughly 80% of all cases. Minor ulcers are small (under 10mm across), oval or round, with a yellow-gray center and a distinct red ring around the edge. They heal without scarring in 7–14 days and usually show up 1–5 at a time.
Major Aphthous Ulcers (Sutton's Disease)
Larger (over 10mm), deeper, and much more painful than minor ulcers. Major aphthous ulcers can last 2–6 weeks and often heal with a scar. They can make eating and speaking difficult. This type is more strongly linked to whole-body conditions like Crohn's disease, celiac disease, and HIV (Scully & Porter, 2008 — PMID: 18279969).
Herpetiform Aphthous Ulcers
Named for how they look like herpes sores — but they are not caused by the herpes virus. Herpetiform canker sores show up as clusters of 10–100 tiny (1–3mm) ulcers that can merge into larger irregular patches. More common in women and older adults. Healing time: 7–30 days.
Whichever type you have, a canker sore heals through the same four predictable phases. For a day-by-day breakdown of what to expect, see Stages of Canker Sore Healing.
Canker Sore vs. Cold Sore: Key Differences
This is the most common diagnostic confusion. For a detailed comparison, see Canker Sore vs. Cold Sore.
| Feature | Canker Sore | Cold Sore |
|---|---|---|
| Location | Inside mouth only | Outside lip, around mouth |
| Contagious | No | Yes (HSV-1) |
| Cause | Multifactorial (immune-mediated) | Herpes simplex virus type 1 |
| Appearance | Yellow-gray center, red border | Clustered fluid-filled blisters |
| Prodrome | Tingling/burning 1–2 days prior | Yes — tingling, itching |
| Recurrence trigger | Stress, trauma, nutritional deficits | Stress, UV light, illness |
| Treatment | Anti-inflammatory, pain relief | Antivirals (acyclovir) |
What Causes Canker Sores?
The short answer: we don't fully know, and anyone claiming a single cause is oversimplifying.
The evidence points to a multifactorial immune-mediated mechanism: in genetically susceptible individuals, various triggers activate an abnormal local immune response that damages the oral epithelium. See our full guide: What Causes Canker Sores?
Common contributors include:
- Nutritional deficiencies (B12, iron, folate, zinc)
- Sodium lauryl sulfate (SLS) in toothpaste
- Stress (HPA axis dysregulation)
- Minor oral trauma
- Hormonal fluctuations
- Genetic predisposition (familial clustering observed)
When Should You See a Doctor?
Most canker sores resolve without treatment. Seek evaluation if:
- An ulcer persists beyond 3 weeks
- Ulcers are unusually large (major aphthous territory) or extremely painful
- You have high fever accompanying the outbreak
- You experience unexplained weight loss, fatigue, or other systemic symptoms (possible Crohn's, celiac, or HIV association)
- Outbreaks are becoming more frequent or severe over time
Need a professional for this? We'll connect you with a dentist or oral medicine specialist in your area who treats recurrent canker sores.
Get connected with local help →The Bottom Line
Canker sores are poorly understood, common, and not dangerous for most people. The primary goals of treatment options are reducing pain and healing time — not curing a root cause, because no single root cause has been established. If you're a chronic sufferer, understanding your triggers is more actionable than searching for a universal cure. See the supplements guide for nutritional deficiencies worth screening, and home remedies for what the evidence actually supports.