I asked my dentist about that once, because I have always loved to drink water with like a whole lemon squeezed in it, VERY tart. Anyway, he said that (a drink) should be okay, but that BITING INTO lemons will ruin your teeth. (some people like to eat lemons straight). So the Master-Cleanse
should be okay, just follow the drink with a bit of plain water swished around, or better yet brush your teeth as often as possible, even if it is with just water.