Aim– The purpose of this clinical trial is to evaluate the adjunctive clinical effects of the systemic administration of ornidazole(ORN) in the full–mouth scaling and root planning (SRP) of individuals with moderate–to–advanced chronic periodontitis. Methods: 50 individuals presenting ≥12 teeth with probing depth (PD) ≥4 mm were selected. All participants were instructed on strict oral hygiene measures and were advised to use 0.2% chlorhexidine mouthwash for 1week before being allocated to two groups. 25 participants were randomly assigned to full–mouth SRP+ placebo (control group), and 25 participants were assigned to full–mouth SRP+ORN (testgroup). The clinical outcomes evaluated were plaque index, gingival index, clinical attachment level (CAL) and PD (Probing Depth). Results: Fifty participant should be evaluated by ≤6months. At 6 months, the test group had greater mean reduction (2.84 mm) in PD(Probing Depth) compared to the control group (0.84 mm) (P <0.05), and there was also a greater mean CAL reduction (2.92 mm) in the test group compared to the control group (0.92 mm) (P <0.05). Conclusion: The systemic use of Ornidazole, when used in conjunction with initial periodontal treatment consisting of SRP in adults with periodontitis, achieves significantly better clinical results than initial periodontal treatment alone.