• A burning or tingling sensation before other symptoms develop (warning symptom)
• Skin lesion on mucous membranes of the mouth
  • Begins as a red spot or bump
  • Then develops into an open ulcer
    • Usually small, 1-2 mm to 1 cm
    • Occasionally larger than 1 cm
    • Single or clusters, often appears in groups (crops)
  • Painful
  • Center appears white or yellow with fibrous texture
  • Halo border is bright red (a red halo may not always be present)
  • May have gray membrane just prior to healing

• Fever
• General discomfort or uneasiness (malaise)
• Swollen lymph nodes

A diagnosis is based on the distinctive appearance of the lesion. If canker sores persist or continue to return, a health care provider or dentist should be consulted to rule out other causes of mouth ulcers. (These include erythema multiforme, drug allergies, herpes simplex infection, pemphigus, pemphigoid, bullous lichen planus, and other disorders.)

Squamous cell carcinoma may first appear as a mouth ulcer that does not heal, but this is rare. A relatively new ulcer-like condition is hairy leukoplakia, a condition that occurs with HIV infection and may be mistaken for the common canker.

A skin lesion biopsy may be used to differentiate a canker sore from other causes of mouth ulcers.