The severe tissue destruction associated with periodontitis may be caused by a deranged antioxidant state. Smoking contributes significantly to the oxidative stress of periodontal tissues in addition to periodontitis. Therefore, this study aimed to investigate and compare the levels of glutathione reductase (GR) in the saliva of smokers and non-smokers with and without periodontitis after non-surgical periodontal treatment (NSPT). Smokers with periodontitis (n = 19), smokers without periodontitis (n = 19), non-smokers with periodontitis (n = 19), and non-smokers without periodontitis (n = 19) comprised the four groups of 76 participants. Clinical measures were measured at baseline, on the 14 day, and one month later. These included plaque index (PI), gingival index (GI), probing pocket depth (PPD), and clinical attachment level (CAL). ELISA was used to measure salivary GR levels before and after NSPT. Improvements in PI and GI scores were associated with NSPT. Following NSPT, there was a significant decrease in PPD and an increase in CAL (P < 0.001). According to intra-group comparison, the GR activity of each study group increased significantly from baseline to one month following NSPT (P < 0.001). The GR activities of the study groups did not differ significantly in the between-group comparison (P < 0.188). The results of this study suggest that NSPT and better dental hygiene were successful in increasing salivary GR levels and reducing oxidative stress. Therefore, salivary GR levels may be regarded as a predictive indication when assessing the periodontal health of smokers and patients with periodontitis.
