TAC1 and periodontitis: Our review included studies of any design (e.g., randomized controlled trials, cohort studies, case-control studies, cross-sectional studies) that compared and evaluated the presence of any one of the neuropeptides such as SP, calcitonin gene-related peptide (CGRP), neurokinin A (NKA), neuropeptide Y (NPY), and vasoactive intestinal polypeptide (VIP) in systemically healthy patients with and without periodontitis.