Canker Sore vs. Chancre: Key Differences in Etiology and Presentation

Canker sores (aphthous ulcers) and chancres are both oral/genital lesions but differ fundamentally in cause, appearance, and significance.

Feature	Canker Sore (Aphthous Ulcer)	Chancre (Primary Syphilis)
Cause	Non-infectious: stress, trauma, food sensitivity, immune factors, nutritional deficiencies	Infectious: Treponema pallidum (syphilis spirochete), sexually transmitted
Location	Oral mucosa: cheeks, lips, tongue, soft palate, gums	Genitals, anus, rectum, lips, mouth (site of inoculation)
Appearance	Small (3–10 mm), round/oval, yellow-gray center with red halo	Single (usually), firm, painless, clean-based ulcer with indurated edges
Pain	Often painful, especially with eating/talking	Typically painless
Number	Single or multiple (crops)	Usually solitary
Healing	7–14 days without scarring	3–6 weeks even without treatment; heals but infection persists
Systemic symptoms	None	May have regional lymphadenopathy (non-tender, rubbery)
Contagious	No	Highly infectious

Clinical Significance

• A canker sore is a benign, self-limiting nuisance.
• A chancre is the hallmark of primary syphilis—a systemic infection requiring urgent antibiotic treatment (penicillin) and partner notification. Untreated, it progresses to secondary, latent, and tertiary syphilis with severe complications.

When to Seek Care

• Any genital ulcer: test for syphilis (RPR/VDRL + confirmatory), herpes, chancroid.
• Oral ulcer persisting > 2 weeks, unusually large, or recurrent: evaluate for underlying causes (celiac, IBD, Behçet's, malignancy).
• If sexual exposure risk exists, do not assume an oral ulcer is "just a canker sore"—get tested.

Accurate differentiation is critical: one needs reassurance and symptomatic care; the other demands immediate antimicrobial therapy and public health intervention.