Allicin (external - login to view)!
Now, it is peculiar that it only occurs when tissue damage occurs. So one way to reduce the amount of allicin, is to chop your garlic into 2-3 pieces only. I was told by an Italian colleague of mine that another method is to chop the garlic and leave it in the fridge overnight. One can also place garlic in oils and store it for a length of time.
If you suffer from heartburn or flatulence after eating, it too may be caused by allicin (external - login to view). So beware your finely chopped or crushed garlic. You may be better off putting it in whole.