Tea is a natural source of fluoride and at moderate levels is regarded as good for teeth. This article from the New England Journal of Medicine describes a 47-year-old woman who used 100 to 150 tea bags every day for over 17 years. The woman was ingesting ~20mg of fluoride every day which resulted in complete tooth loss and bones that were seven times denser than normal. Aside from tooth-loss, the fluorosis led to severe brittle bones and pain.
Everything in moderation….