I have suffered from canker sores all my life and have narrowed down chocolate, coke as two of the causes. I also get them when I am just run-down. I also get them for no apparent reason at all. I thought that maybe I wasn't getting enough of some vitamin -- so I bought a multi-vitamin supplement. It ended up giving me canker-sores. I recalled how one time previously, an anti-biotic I was taking [for something entirely different] -- also gave me canker-sores, which at the time I thought was a function of being run-down. My question is: Is it possible to have an allergic reaction to the coating on a pill? I've also seen posted in some places the suggestion that canker-sores are caused by some type of yeast build-up in the digestive system, and anti-biotics can throw off the natural order of things. Any wisdom on this subject would be hugely appreciated.