from http://www.mayoclinic.com/health/canker-sore/DS00354/DSEC...
"The precise cause of canker sores remains unclear, though researchers suspect that a combination of several factors contributes to outbreaks, even in a single individual. These factors include:
1) A minor injury to your mouth from dental work, overzealous brushing, sports mishaps, spicy or acidic foods, or an accidental cheek bite
2) Food sensitivities, particularly to chocolate, coffee, strawberries, eggs, nuts, cheese and highly acidic foods, such as pineapple
3) Food allergies
4) A diet lacking in vitamin B-12, zinc, folate (folic acid) or iron
5) An allergic response to certain bacteria in your mouth
6) Helicobacter pylori, the same bacteria that cause peptic ulcers
7) Celiac disease, a serious intestinal disorder caused by a sensitivity to gluten, a protein found in most grains
8) Inflammatory bowel diseases, such as Crohn's disease and ulcerative colitis
Behcet's disease, a rare disorder that causes inflammation throughout the body, including the mouth
9) A faulty immune system that attacks healthy cells in your mouth instead of pathogens, such as viruses and bacteria
10) HIV/AIDS, which suppresses the immune system
11) Hormonal shifts during menstruation
12) Emotional stress
13) Toothpastes and mouth rinses containing sodium lauryl sulfate
lifestyles and home remedies:
To relieve pain and speed healing:
Rinse your mouth. Use salt water; baking soda (dissolve 1 teaspoon of soda in 1/2 cup warm water); hydrogen peroxide diluted by half with water; or a mixture of 1 part diphenhydramine (Benadryl) to either 1 part bismuth subsalicylate (Kaopectate) or 1 part simethicone (Maalox). Be sure to spit out the mixtures after rinsing.
Cover lesions with a paste made of baking soda.
Try over-the-counter products that contain a numbing agent, such as Anbesol and Orajel.
Avoid abrasive, acidic or spicy foods that can cause further irritation and pain.
Apply ice to your canker sores or allow ice chips to slowly dissolve over the sores.
Brush your teeth gently, using a soft brush and toothpaste without foaming agents, such as TheraBreath.
Dab a small amount of milk of magnesia on your canker sore a few times a day. This can ease the pain and may help the sore heal more quickly.
GOOD LUCK!