**6. Schizophrenia**

Evidence for a significant relationship between periodontitis and schizophrenia has not ye<sup>t</sup> been accumulated. Only one cross-sectional study with a small sample size has concluded that patients with schizophrenia have a high risk of periodontitis and there is an even higher risk in those who are taking antipsychotics that reduce salivary secretion and cause xerostomia [73]. Intriguingly, human genome/gene analysis on insertion/deletion (D) polymorphism in the angiotensin-converting enzyme (ACE) gene has indicated that the D allele is a protective factor against schizophrenia [74] and chronic periodontitis [75]. Accordingly, the ACE D allele may be a clue to reveal any biological and reciprocal connection between these two diseases.
