Xylitol is extremely powerful (probably the most effective antimicrobial ever discovered) in its effect on pathogenic bacteria. It doesn’t just kill and eradicate s. mutans, the bug responsible for cavities, but also candida and many pathogens involved in periodontal disease. It has been known to heal hopelessly large periodontal pockets very rapidly and clear up gum disease like nothing else. It is doomsday for bacteria. It tastes sweet like sugar, but should be treated like the powerful medicine it is. Additionally, it appears to help with insulin control and osteoporosis.
Use it sparingly at first. Like many human foods, dogs cannot have it, it can cause fatal hypoglycemia

Here is an interesting site that looks at its dental effects:

http://www.healingteethnaturally.com/rinsing-mouth-brushing-teeth-with-xylitol-sugar.html

Xylitol is a huge threat to the dental industry and even to the health and alternative health industry. Unlike many of the herbs and healing recommendations and diets around, xylitol works and does it fast and cheap.

My husband however, had the experience of increasing rawness in his mouth, which after a couple of weeks got so bad he was forced to start trying to eliminate the cause by stopping one after another of the elements of Dr. Ellie’s regime, beginning with the most caustic rinses (the alcohol based Listerine) and waiting a few days before he changes anything else, to see what the effect is. He is now down to eliminating the most innocuous rinse, the xylitol solution (he’s only been doing one teaspoon to a liter of water), and after only one day off of it, he’s finally feeling a bit of relief, though its too soon to be sure it will heal up completely. A google search of “xylitol adverse reactions” brought up a mention of rare allergic reactions (for the allergic people possible at any dose), and of “oral excema” (rash, raw skin in the mouth) which is only supposed to occur at daily dosages above 60 grams, not the 6-10 grams my husband has been taking. He’s really disappointed, because his teeth could really use the remineralization effect of xylitol.

There are also papers describing the kill-off reactions that others have mentioned above – I suppose what my husband is experiencing could b e a manifestation of that. Xylitol is supposed to have antifungal and anti-yeast properties (thus its worthless for making yeast-raised baked goods). I’m wondering if my husband’s mouth already had fungal or yeast organisms incubating in the skin, if that could explain why his skin is being attacked, and not mine. Or perhaps our biochemistry is just that different; if the average person makes 15 grams of xylitol per day, what happens to a person who makes a lot more than the normal amount in their body and then takes an external dose on top of that – would they get the symptoms of an overdose? We may never know, but each person knows their own body best and must listen to it.

Lastly, I wanted to mention that not all xylitol is created equal (hardwood sourced is best – much of the Chinese stuff comes from agribiz corncobs), and that many products that boast they contain xylitol also contain other sugar substitutes like sorbitol, which definitely can cause gastro-intestinal troubles, and can also cause dental cavities worse than those caused by pure sugar. Those who’ve had bad reactions they’ve blamed on xylitol would do well to find out where it was sourced and whether it was pure or blended with other artificial sweeteners.

In our case we were careful to get the 100% pure hardwood sourced xylitol, yet it didn’t make a difference for my husband. Interestingly, I think he doesn’t have a noticeable bad reaction to aspartame/Nutrasweet, though he avoids it anyways, yet when I get some by mistake, I know it almost immediately, for it gives me a splitting headache in less than 10 minutes, and I’m not prone to headaches unless I’m really dehydrated.