No one knows what causes canker sores, or why women are more likely to get them. Outbreaks often seem related to physical or emotional stress. Unlike cold sores, canker sores are not contagious. They don't pose any health risk other than the discomfort that comes along with them.
I was plagued by constant long lasting painful canker sores my whole life until one day i noticed that a new toothpaste made them even worse. So to test my theory I started using 'baby' toothpaste (by Oragel). It took a while but lo and behold I now rarely get any and if I do they are no longer severe and last a very short time. This has been for years. It's like a miracle!!
The baby toothpaste does not make my mouth feel as fresh as regular toothpaste but if I use a tiny bit of regular toothpaste sometimes it doesn't seem to bother me.
Also I found that rinsing with salt water a few times a day is the best way to treat the canker sores.
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