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.

Reviewed: January 2012