For as long as I can remember, I have bought into the gospel of fluoride, believing that my teeth would surely rot out of my head without its protection. So it felt a little bit illicit, recently, when I purchased a box of German fluoride-free kids’ toothpaste for my daughter. The toothpaste came in blue, understated packaging—no cartoon characters or candy flavors—which I associated with German practicality. And instead of fluoride, it contained an anticavity ingredient called hydroxyapatite, vouched for by several dental researchers I interviewed for this story. Could it be, I wondered as I clicked “Buy,” that toothpaste doesn’t need to contain fluoride after all? The scientific case for hydroxyapatite toothpaste is actually quite simple: Composed of calcium and phosphate, hydroxyapatite is the very mineral that primarily makes up our bones and teeth. Tooth enamel, the hard protective outer layer, is naturally about 96 percent hydroxyapatite. NASA researchers first patented an idea for repairing teeth with a hydroxyapatite precursor in the 1970s; nothing came of it then, but a Japanese company acquired the patent and eventually created a popular toothpaste called Apagard.