**8. Conclusions**

The relationship between periodontitis and neuropsychiatric disorders, in particular AD, has recently attracted researchers' attention, and the evidence for their considerable reciprocal association has been accumulated. Specifically, various clinical and experimental studies imply the potentially causal link of periodontitis to neuropsychiatric disorders, and neuroinflammation seems to be a key pathological connector between them. After establishment of such a link, a broad spectrum of neuropsychiatric disorders associated with neuroinflammation may be preventable and modifiable by simple daily dealings for oral hygiene, even though the notion that periodontitis is a risk factor for neuropsychiatric disorders remains to be effectively substantiated.

**Author Contributions:** S.H. wrote the manuscript. All authors discussed and edited the manuscript. All authors read and approved the final manuscript.

**Funding:** This study was supported by JSPS KAKENHI Grant Numbers 19K08018 (SH), 17K09194 (KI), 19K08046 (MH).

**Acknowledgments:** Sincere appreciation is extended to Edith G. McGeer for her invaluable support.

**Conflicts of Interest:** A.O.-N. has more than 5% shares of RESVO Inc. M.I. received lectures fees from Meiji, Mochida, Takeda, Novartis, Yoshitomi, Pfizer, Eisai, Otsuka, MSD, and Sumitomo Dainippon, personal fees from Technomics and research funds from Novartis, Eisai, Astellas, Pfizer, Daiichi-Sankyo, Takeda, and MSD. The funders had no role in the study design, decision to publish, or preparation of the manuscript.
