Objectives Essential oils and other plant extracts have evoked interest as sources of natural medicinal products. They have been proven to exert antibacterial, antifungal, antiviral and antioxidant properties, but the mechanism of action has not been fully elucidated. Aim This study aims to evaluate the potential of a sage-containing mouthwash to alleviate inflammatory signs of intra-oral mucosa and gingiva in comparison with a water/alcohol-based placebo. Material and methods This study was conducted as a randomised, placebo-controlled, double-blind, parallel design clinical study. Forty-eight dentate subjects were randomly assigned to a test (sage-containing mouthwash) or a placebo group (water/ alcohol-based solution). Subjects rinsed once daily for 30 s over a period of 6 weeks. Sulcus Bleeding Index (SBI), Plaque Index (PLI), tooth staining, xerostomia and degree of stomatitis were assessed at baseline and after 6 weeks. Results Subjects' mean age was 77.5 ± 7.3 years. SBI was reduced from 1.3 ± 0.9 to 0.8 ± 0.7 (test, p = 0.0029) and 1.4 ± 0.9 to 1.1 ± 0.7 (placebo, p = 0.0105). Similarly, PLI was reduced from 1.2 ± 0.5 to 1.0 ± 0.3 (test, p = 0.0080) and 1.3 ± 0.4 to 1.1 ± 0.6 (placebo, p = 0.0087); no between-group differences were found (p > 0.05). Stomatitis, xerostomia and tooth staining revealed no change after 6 weeks. Conclusion The irrigation with a sage-containing mouthwash did not result in a superior beneficial effect on inflammatory parameters and plaque indices compared with the placebo. The expected contribution of the plant extracts to their potential impact on oral health may need further investigation. Clinical relevance The proposed mouthwash might be suitable for patients with inflammatory signs of the gingiva who prefer natural remedies.