Canker & Cold Sores

Canker or Cold Sore?

Provided by Academy of General Dentistry

Where it is tells you what it is and what to do about it

In or out? That's the first question to ask when you're trying to figure out whether you have a canker sore or a cold sore. If it's inside your mouth it's most likely a canker sore; outside, probably a cold sore.

The two may seem similar, but the similarity ends with the fact that both are connected with the mouth and both cause pain and discomfort. So knowing which one you have is the first step to knowing how it's caused and how to treat it.

Canker sores (or mouth ulcers) generally occur inside the mouth and are bacterial in nature. They often are triggered by trauma such as biting your cheek, jabbing your gum with your toothbrush or even overzealous tooth cleaning. Cold sores are tiny, clear, fluid-filled blisters that form around the mouth and are caused by the herpes simplex virus living inside your nerve tissue. Below is a table to help you identify the two different sores.

Question Cold Sore Canker Sore
1. Where does it strike? Outside the mouth Inside the mouth
2. What does it look like? Tiny, clear fever blisters usually on the lips or under the nose Small ulcer with a white or gray base and red border
3. What triggers it? Flare-up of herpes simplex virus Trauma-Biting your cheek or jabbing gum witih toothbrush
4. How long does it last? About a week One or two weeks
5. Is it contagious? Yes No
6. How do I treat it?
  • Aloe Vera or over-the-counter topical anesthetics to cut pain and discomfort
  • Over-the-counter medications or, if needed, prescription antiviral drugs to prevent outbreak
  • Rinse with antimicrobial mouthwash or warm water and salt
  • Over-the-counter oral anesthetics

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