This is the single most common confusion in oral ulcer diagnosis, and it matters because the treatments are completely different — and some treatments for one actively worsen the other.
The One-Sentence Distinction
Canker sore: inside the mouth, not contagious, immune-mediated, no virus.
Cold sore: outside the lip or around the mouth, contagious, caused by herpes simplex virus type 1 (HSV-1).
Clinical Differences
Location
This is the most reliable quick differentiator:
Canker sores form exclusively on movable mucosa — the soft tissues that are not attached to bone:
- Inner cheeks (buccal mucosa)
- Inner lips
- Tongue (tip, sides, underside)
- Soft palate
- Floor of mouth
- Base of gums (not the gum tissue attached to teeth)
Cold sores form on keratinized tissue and the external lip:
- The vermilion border (the outer edge of the lip)
- Around the mouth on facial skin
- Less commonly: inside the mouth on hard palate or attached gingiva (in immunocompromised patients)
If it's inside the soft tissue of the cheek: canker sore. If it's on the outer lip: cold sore.
Appearance
Canker sore:
- Round or oval
- Yellow-gray fibrinous center
- Distinct red halo
- Single or small cluster (1-5 ulcers)
Cold sore:
- Begins as grouped fluid-filled blisters (vesicles)
- Blisters rupture to form a crust
- No fibrinous center — weeping, then crusted
Prodrome
Cold sores have a reliable prodrome: 1-2 days of tingling, burning, or itching at the site before the blister appears. This is HSV reactivating from its latent state in the trigeminal ganglion.
Canker sores may have a vague burning sensation before appearing, but there's no tingling-then-blister sequence. The ulcer seems to appear more suddenly.
Contagiousness
Cold sores: Highly contagious. HSV-1 is transmitted via direct contact — kissing, shared utensils, oral contact during a breakout. Approximately 67% of adults under 50 have HSV-1 antibodies (WHO data), most acquired in childhood.
Canker sores: Not contagious at all. There is no infectious agent to transmit.
Recurrence Triggers
Both recur, but triggers differ:
| Trigger | Canker Sore | Cold Sore | |---|---|---| | Stress | Yes | Yes | | Fever / illness | Rarely | Yes — "fever blister" | | Sun exposure | No | Yes | | Minor oral trauma | Yes | No | | Nutritional deficiency | Yes | No | | Hormonal cycles | Yes (some patients) | Less consistent |
Why the Confusion Matters for Treatment
Antivirals Do Nothing for Canker Sores
Acyclovir, valacyclovir, famciclovir — these are antiviral drugs that inhibit HSV replication. They are effective for cold sores. They do nothing for canker sores because there is no virus to inhibit.
This is not just a money issue. Applying antiviral cream to a canker sore is equivalent to applying antibiotic cream to a bruise. You're solving for the wrong mechanism.
Topical Steroids Work for Canker Sores, Not Cold Sores
Topical corticosteroids (fluocinonide, triamcinolone) reduce the T-cell-mediated inflammation driving canker sore tissue destruction. They speed healing meaningfully.
For cold sores, topical steroids can actually worsen the outbreak by suppressing the immune response that controls viral replication. Applying steroid cream to a cold sore risks spreading the lesion.
Quick Reference
| | Canker Sore | Cold Sore | |---|---|---| | Medical term | Aphthous ulcer / RAS | Herpes labialis | | Cause | Immune dysregulation | HSV-1 (rarely HSV-2) | | Location | Inside mouth, soft tissue | Outside lip, facial skin | | Contagious | No | Yes | | Appearance | Yellow-gray center, red border | Fluid blisters → crust | | Tingling prodrome | No | Yes | | Treatment | Anti-inflammatories, analgesics | Antivirals (acyclovir) | | Prevented by antivirals | No | Yes (suppressive therapy) |
When You're Still Not Sure
If the location is ambiguous (e.g., the inner vermilion border), or if you're getting frequent outbreaks and genuinely unsure which type they are: a dentist or oral medicine specialist can diagnose definitively. For cold sores, HSV swab testing during an active outbreak is definitive. For canker sores, there's no diagnostic test — it's a clinical diagnosis of exclusion.