No safety concerns have been reported for hydroxyapatite used in toothpaste.[1] In supplements taken by mouth, ossein-hydroxyapatite has been used in studies for up to four years without safety issues, while microcrystalline hydroxyapatite has been used for up to one year.[2][3] Common adverse effects are gastrointestinal (constipation, dyspepsia, abdominal pain, vomiting, etc) and seem to occur at a slightly lower rate than with calcium carbonate supplements.[4][2] Since hydroxyapatite dietary supplements are typically from bovine sources, improper manufacturing processes might increase the risk of variant Creutzfeldt-Jakob disease(vCJD), also known as “human mad cow disease”.[5][6][7]