+1 for mastic, completely nontoxic and it's been used for thousands of years. My cavity problems have pretty much disappeared since discovering it several years ago.
For someone trying it for the first time, I would recommend the Falim chewing gum, available on Amazon, or the mint-flavored toothpaste at mastic.gr. If you feel comfortable trying pure mastic, the best quality that I've found are the large tears from Chios Mastiha Growers Association.
