Canker sores are small, painful ulcers in the mouth that can make chewing or smiling uncomfortable. While irritating, however, canker sores are neither dangerous nor contagious.

Much of the confusion surrounding canker sores stems from confusion about the difference between “canker sores” and “cold sores.” While these two terms are often used interchangeably with each other, they are far from the same thing.

A cold sore is a fluid-filled blister that typically appears outside the mouth, around the lips. Canker sores, meanwhile, only appear inside the mouth, usually on the cheeks or tongue. And while cold sores are brought on by a virus, canker sores are caused by stress or tissue damage in the mouth. Cold sores are highly contagious; canker sores are not. If you get a cold sore it’s best to refrain from close contact with others. But if you have a canker sore, don’t sweat stealing a smooch from your sweetheart.

Canker sores tend to reappear periodically and preventing them can prove difficult. However, if you are prone to canker sores, it’s a good idea to avoid acidic foods that may irritate the mouth. Also make sure that you refrain from chewing on the inside of your mouth, as this habit can lead to the sort of minor tissue damage that gives rise to canker sores.