The raw lemon juice won't hurt your teeth so long as you're still brushing them. Don't use toothpaste though as the taste can stimulate the digestion system which takes energy away from the real jobs the body needs to do during a fast. Which brings me to my advice about the lemon juice.. best to not have it during a Water Fast at all as it will certainly interfere with the fast. It is a great way to break your fast however. The only safe thing to use if you want a feeling of fullness is pure carbonated water. There is no safe way to continue tasting while you are fasting.