I used to get canker sores occasionally, almost like clock work. I did some digging around and found out that a common foaming agent in most toothpastes - Sodium Lauryl sulphate (sp?), is known to cause recurring canker sores in some people. The thing that sucks, however, is that this stuff is used in almost every type of tooth paste. I asked my dentist about it, and he said the stuff is added to toothpaste to provide more foaming, but doesnt necessarily have a big impact on the effectiveness of toothpaste.
The only readily available toothpaste I have found that doesnt have SLS is Rembrant Complete. Its in a white box, with green lettering. I used to get canker sores every month or so. Since switching to this toothpaste I havent had a canker sore in a little over two years.
edited to add a link:
http://www.acankersorecure.com/cure-...ker-sores.html
I was skeptical when I switched away from toothpastes with SLS in it, but it seriously seems to have done the trick for me.