The first symptom is usually a tingling or burning sensation that you feel before other symptoms develop.

The following symptoms may then occur:

• Painful, red spot or bump that develops into an open ulcer
  • Center is colored white or yellow
  • Usually small (under 1 cm) but occasionally larger
  • Single bump or group of bumps (crops)
• Sore may turn gray just before starting to heal

Less common symptoms include:

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

Pain decreases in 7 to 10 days, with complete healing in 1 to 3 weeks. Particularly large ulcers (greater than 1 cm in diameter) often take longer to heal (2 to 4 weeks). Occasionally, a severe occurrence may be accompanied by nonspecific symptoms of illness, such as fever. Canker sores often return.

Your health care provider can often make the diagnosis by looking at the sore. If canker sores persist or continue to return, tests should be done to rule out other causes, such as erythema multiforme, drug allergies, herpes infection, bullous lichen planus, and other disorders.

Canker sores are not cancer and do not cause cancer. There are types of cancer, however, that may first appear as a mouth ulcer that does not heal. See: Squamous cell carcinoma.

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