Complex sentence: By 2000, however, the topical effects of fluoride (in both water and toothpaste) were well understood, and it had become known that a constant low level of fluoride in the mouth works best to prevent cavities.
Split into simpler sentences: By 2000, however, the topical effects of fluoride (in both water and toothpaste) were better understood. The current dental position is that a constant low level of fluoride in the mouth works best to prevent cavities.