Short answer

Mouth sores are overwhelmingly non-STI, canker sores, cold sores, bites, braces trauma, viral illnesses, with syphilis and herpes as the STI exceptions that exposure history plus lesion character identify.

Most heal within two weeks regardless of cause.

Persistent, painless, solitary, or exposure-linked sores need clinician evaluation with STI testing; typical painful canker patterns need only watchful comfort care.

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Common causes. The vast majority

  • Aphthous (canker) ulcers: sharply painful, inside the mouth (not lip borders), gray-white centers with red halos, healing in 1–2 weeks.
  • HSV-1 cold sores: clustered blisters on lip borders and adjacent skin, often with prodromal tingling, recurrent at the same sites.
  • Traumatic ulcers (bites, braces, sharp foods, dental work).
  • Geographic tongue (migrating map-like patches. Benign, chronic).
  • Hand-foot-mouth disease (children and close contacts. Mouth plus hand/foot lesions).
  • Medication reactions (lichenoid patterns, dryness-related ulceration).

These explain the overwhelming majority of mouth sores seen in primary care.

STI patterns. The exceptions with signatures

  • Syphilis: painless solitary lip or tongue chancre with firm rolled edges plus swollen regional nodes. The painlessness distinguishing it from nearly every other sore.
  • Herpes: painful lip or intraoral clusters, sometimes severe in first episodes (gingivostomatitis with fever and swollen gums), milder in recurrences. Both follow oral sexual exposure by days to weeks.
  • Gonorrhea rarely ulcerates orally (sore throat patterns instead. See throat testing guidance).

Exposure history plus lesion examination sorts STI from non-STI; coinfection workups (syphilis plus HIV screening) accompany STI-attributed sores.

Route context at How Do Sexually Transmitted Infections Spread.

Red flags beyond STIs. Biopsy and urgency thresholds

  • Non-healing ulcers beyond 2–3 weeks (oral cancer territory. Biopsy, not watchful waiting, especially with tobacco/alcohol history).
  • White or red patches that don’t wipe off (leukoplakia, erythroplakia. Premalignant until proven otherwise).
  • Thrush or hairy leukoplakia with immune suppression (antifungal plus HIV-evaluation contexts).

These thresholds apply regardless of sexual history. Persistent mouth lesions earn tissue diagnosis on schedule.

Evaluation. What the visit runs

  • Lesion examination mapping number, pain, borders, and nodes.
  • Herpes NAAT swabs for vesicular or ulcerative lesions (fresh beats healed for yield).
  • Syphilis serology for painless ulcers, rashes, or relevant exposures.
  • HIV screening by risk profile. Dental or oral-surgery referral for persistent lesions needing biopsy.

Present early in the lesion’s life. Diagnostic yield decays with healing daily.

When to seek care

Promptly for sores with oral-sex exposure, painless solitary ulcers, or recurrent clusters; urgently for eye-area spread, inability to swallow, or immune suppression with severe lesions.