If you use something to treat your teeth, it is topical, in that sense. You don’t drink teach whitening bleach. You don’t drink salicylic acid for dandruff! That’s my point. I don’t know why drinking fluoride is so important in this day and age.
It's the difference between treating the surface and strengthening the structure from the inside out.
Think of the tooth as a bone...and the importance of diet to the bone's structure.