TL;DR
In everyday use, "mouth ulcer" and "canker sore" mean the same thing — both describe the painful, round sores that appear on the soft tissues inside the mouth and resolve on their own in 1–2 weeks. Technically, "mouth ulcer" is a broader category. A canker sore specifically refers to an aphthous ulcer — an immune-mediated oral ulcer with a specific morphology and mechanism. Other types of mouth ulcer (caused by trauma, viral infection, medication, or systemic disease) look similar but have different causes and management. If you have typical-looking, recurrent sores on the soft tissues inside your mouth, they're almost certainly canker sores. If the pattern is atypical — wrong location, unusual appearance, not healing, or accompanied by systemic symptoms — a different diagnosis is worth considering.
When They Mean the Same Thing
If you're describing a round, painful white or yellow sore with a red border that appeared inside your mouth, isn't on the outer lip, doesn't have blisters, and resolves in 1–2 weeks — you have a canker sore, and "mouth ulcer" and "canker sore" are interchangeable terms for it.
"Canker sore" is the common American term. "Mouth ulcer" is more common in the UK, Australia, and parts of Canada. "Aphthous ulcer" is the clinical/medical term. "Aphthous stomatitis" refers to the recurring condition. All four are describing the same entity.
The Technical Distinction: What Makes It a Canker Sore Specifically
A true canker sore (aphthous ulcer) has specific defining features:
Morphology:
- Round or oval ulcer, rarely irregular in shape
- Central pseudomembrane: white, yellow, or gray base with a fibrinous coating
- Raised erythematous (red) halo around the edge
- Painful — often disproportionately so relative to its size
Location — always on non-keratinized mucosa:
- Inner cheeks (buccal mucosa)
- Inner lips (labial mucosa)
- Tongue (lateral surface, underside, tip — rarely dorsal)
- Floor of mouth
- Soft palate
- Base of gums (attached gingiva/sulcus)
What they are not:
- Not blistering — no fluid-filled vesicle precedes them
- Not on the outer lip or vermilion border
- Not on the hard palate (roof of mouth) or attached gingiva in most cases — those locations are more typical of herpes stomatitis
- Not contagious
Mechanism: Aphthous ulcers are immune-mediated — CD8+ T-cells attack the oral mucosal lining through a misfiring immune response. They are not caused by a virus, bacteria, or fungal infection.
Other Types of Mouth Ulcer That Aren't Canker Sores
"Mouth ulcer" used loosely can also describe these conditions, which have different causes and management:
Traumatic Ulcers
Caused by physical injury to the mouth: biting the cheek, sharp food (chips, crust), a rogue toothbrush bristle, orthodontic brackets, or ill-fitting dentures. These are single ulcers at a clear injury site, often irregular in shape, and resolve once the trauma stops. They don't recur on a pattern unless the source of trauma persists.
Distinction from canker sores: Single incident, clear cause, irregular edges, resolves and doesn't recur.
Herpes Stomatitis (HSV-1)
Primary HSV-1 infection in children causes gingivostomatitis — diffuse painful ulcers throughout the mouth, often accompanied by fever, swollen glands, and significant systemic illness. Recurrent HSV-1 in adults produces cold sores on the outer lip, or occasionally intraoral ulcers on the hard palate or attached gingiva (keratinized tissue).
Distinction from canker sores: Preceded by blisters (vesicles) that rupture to form ulcers; keratinized mucosa location (hard palate, attached gum); may have systemic symptoms in primary infection; responds to antivirals.
Hand-Foot-Mouth Disease
A viral illness (Coxsackievirus A16, Enterovirus 71) common in children under 5. Produces painful oral ulcers on any mucosal surface alongside characteristic skin lesions on palms, soles, and sometimes buttocks. Fever is typical.
Distinction from canker sores: Associated skin lesions; systemic illness; multiple oral ulcers appearing simultaneously; primarily affects children.
Drug-Induced Ulcers
Several medications cause oral ulceration as a side effect: methotrexate (antimetabolite, antifolate effect), nicorandil (antianginal), NSAIDs, chemotherapy agents, and others. These appear during medication use and resolve when the medication is stopped or the dose adjusted.
Distinction from canker sores: Temporal correlation with starting a medication; may not have the typical round aphthous morphology.
Erosive Lichen Planus
An autoimmune condition causing irregular, painful erosions on the buccal mucosa and sometimes gingiva. Often presents with a lace-like white pattern (Wickham's striae) around the erosions. Does not spontaneously resolve like canker sores — it's a chronic condition requiring management.
Distinction from canker sores: Irregular erosive morphology rather than discrete round ulcers; Wickham's striae; chronic course without spontaneous resolution; affects gingiva.
Behçet's Disease
Severe recurrent oral ulcers that look identical to canker sores — but occur alongside genital ulcers, eye inflammation (uveitis), and skin lesions. See Canker Sores and Behçet's Disease.
Distinction from canker sores: Systemic features (genital ulcers, eye involvement, skin lesions). Oral ulcers alone cannot distinguish Behçet's from RAS.
Oral Cancer (Squamous Cell Carcinoma)
An oral ulcer that doesn't heal within 3 weeks, has indurated (firm, raised) borders, is painless, or is located on the posterior lateral tongue, floor of mouth, or soft palate warrants clinical evaluation. These are not "mouth ulcers" in the common sense — but a cancer lesion can be mistaken for a persistent canker sore.
Distinction from canker sores: Non-healing beyond 3 weeks; indurated edges; painless; unusual location; doesn't follow the pattern of prior outbreaks.
How to Tell If What You Have Is a Canker Sore
Likely a canker sore if:
- Round or oval, with a white/yellow center and red halo
- Located on soft tissue (inner cheek, lip, tongue, soft palate, floor of mouth)
- Painful from the start — not painless
- Healing within 2 weeks
- You've had them before, possibly since childhood
- No systemic symptoms
Worth investigating further if:
- Doesn't heal within 3 weeks
- Located on the hard palate, outer lip, or firmly attached gum tissue (more typical of herpes)
- Irregular shape or indurated (firm, raised) border
- Preceded by blisters
- Accompanied by fever, swollen glands, or genital sores
- Significantly larger than usual (>10mm) or numerous clusters