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 documented in medical literature since antiquity, yet its exact cause remains incompletely understood.
Prevalence: Approximately 20% of the general population experiences recurrent aphthous ulcers at some point in their lives, making it the most common oral mucosal condition. Prevalence is higher in higher socioeconomic groups, likely due to reporting and diagnosis bias rather than true biological difference.
The Three Types of Canker Sores
Minor Aphthous Ulcers
The most common form, accounting for roughly 80% of all cases. Minor ulcers are small (under 10mm in diameter), oval or round with a yellow-gray fibrinous base and a distinct red halo. They heal without scarring in 7–14 days. They typically occur in crops of 1–5 at a time.
Major Aphthous Ulcers (Sutton's Disease)
Larger (over 10mm), deeper, and significantly more painful than minor ulcers. Major ulcers can persist for 2–6 weeks and frequently heal with scarring. They may interfere with eating and speaking. Major aphthous is associated more strongly with systemic conditions like Crohn's disease, celiac disease, and HIV (Scully & Porter, 2008 — PMID: 18279969).
Herpetiform Aphthous Ulcers
Named for their resemblance to herpetic lesions (not caused by herpes simplex virus). Characterized by crops of 10–100 tiny (1–3mm) ulcers that may coalesce into larger irregular lesions. More common in women and older age groups. Healing time: 7–30 days.
Canker Sore vs. Cold Sore: Key Differences
This is the most common diagnostic confusion.
| 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
The Bottom Line
Canker sores are poorly understood, common, and not dangerous for most people. The primary goals of treatment 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.