Canker sores, also called aphthous ulcers, are small, painful sores that appear inside the mouth on the lips, cheeks, on the gums, and tongue.

They are appropriately named, too: In Greek, aphthae (root of aphthous) means "to set on fire."

Most of us get canker sores from time to time; some of us get them over and over.

Either way, you want to get rid of them as quickly as possible. We talked to doctors to find out where canker sores come from, how to avoid them, and what to do if you get one.

No one knows exactly what causes most canker sores, but we know they’re not contagious and can’t be spread through saliva.

They can be the result of an injury, like when you accidentally bite your cheek or when braces catch or rub against the inside of the cheeks or back of the lips, but often they appear seemingly out of nowhere.

They can be triggered by a number of things, like a weak immune system, dehydration, allergies, stress, and even medication.

Certain medical conditions may also cause canker sores. Inflammatory bowel diseases like ulcerative colitis or Crohn's disease can be a culprit. People with herpes and HIV/AIDS may get canker sores because their immune systems are more vulnerable.

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Unlike cold sores, canker sores don’t occur on the external surface of your lips (outside the mouth).

"Even though canker sores and cold sores can have the same triggers, canker sores are not contagious," says Dr. Varinthrej Pitis. "There is no virus or bacteria associated with them. They tend to go away on their own within 10 days, and they’re not life-threatening."

Avoid canker sores with these tips:

• Brush and floss after meals to keep your mouth clean and free of excess foods.
• Use a soft toothbrush to prevent irritation to your mouth and gums.
• Use orthodontic waxes to cover up any sharp edges on braces or other dental devices.
• Practice stress-reduction techniques, such as meditation.
• Keep a food diary. You may find connections between when canker sores appear and what you eat.

"I’d also stay away from mouthwash," says Dr. Nitin Kapur, primary care physician. "I get we all like that minty taste, but mouthwash is alcohol-based and that can cause a lot of irritation."

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If the canker sore is unusually large, lasts 2 weeks or more, extends to the lips, makes it very difficult to eat or drink, or if you develop a fever, seek the advice of your primary care doctor, a dentist, or dermatologist.

"Most people who get reoccurring canker sores will get them 2-3 times a year, on and off, starting by the time they’re 30 years old," says Dr. Kapur. "But if you're older and getting them for the first time, that may be an indication something else is going on, so you’ll want to seek medical advice."