There are no warnings regarding the actual safety of hydroxyapatite, although any surgery or foreign material being inserted into the body carries risks. Biocompatibility studies on hydroxyapatite implants have all been characterized by inflammatory response, with no foreign body response and with no significant giant cell reactions. There has been no evidence of local or systemic toxicity. Reasons for its good biocompatibility is partly due to the metabolites of these hydroxyapatite implants are calcium and phosphate ions. Another reason is that hydroxyapatite implants share a unique ability to become directly bonded to the surrounding bone without the presence of an intervening fibrous shell. This bonding contributes to the excellent biocompatibility and in large part to the clinical utility of these implants [3]. Hydroxyapatite implants from sea coral are totally non-immunogenic and do not provoke allergic reactions because of the high heat process used to create the structures. In addition to solid hydroxyapatite implants, there is also a hydroxyapatite injectable (or spreadable) paste. Unlike solid hydroxyapatite implants the paste is non-porous so bone and soft-tissue ingrowth does not occur [4].

Implants of hydroxyapatite are usually well tolerated. Some discomfort directly after the implantation should be expected. There may be some swelling, aching, and numbness but these usually resolve after a week or so. Some complications may rarely occur include infection of the implant site, extrusion, induration, seroma or inadequate healing [15].

References

[1] https://carifree.com/2018/07/19/common-uses-of-hydroxyapatite-and-how-it-can-affect-your-teeth/
[2] D. Loca, J. Locs, A. Dubnika, V. Zalite, L. Berzina-Cimdina, Porous hydroxyapatite for drug delivery,  Hydroxyapatite (HAp) for biomedical applications, Elsevier2015, pp. 189-209.
[3] https://www.dermnetnz.org/topics/hydroxyapatite-implant/
[4] https://carifree.com/2018/07/19/common-uses-of-hydroxyapatite-and-how-it-can-affect-your-teeth/
[5] https://www.dermnetnz.org/topics/hydroxyapatite-implant