Stress or minor injury to the inside of the mouth is thought to be the cause of simple canker sores.
Certain foods including citrus or acidic fruits and vegetables (such as lemons, oranges, pineapples, apples, figs, tomatoes, strawberries) can trigger a canker sore or make the problem worse.
Some ways to get rid of canker sores include gargling with salt water, gargling with apple cider vinegar, applying some hydrogen peroxide to the canker sore etc.
To help relieve pain and speed healing of a canker sore or mouth ulcer, consider these tips: Rinse your mouth.
Use salt water or baking soda rinse (dissolve 1 teaspoon of baking soda in 1/2 cup warm water).
Dab a small amount of milk of magnesia on your canker sore a few times a day.