Aim This 3-month double-blind randomized placebo-controlled study evaluated the clinical and microbial effects of an essential oil mouth rinse used as an adjunct to mechanical plaque control by patients in supportive periodontal care. Material and methods Fifty patients were randomly allocated to an essential oil group (Listerine® Coolmint; Johnson & Johnson, New Brunswick, NJ, USA) or placebo group to rinse twice per day as an adjunct to mechanical plaque control. At baseline and after 3 months, plaque index (PI), gingivitis index (GI), probing pocket depth, bleeding on probing (BoP) and clinical attachment level were registered. Subgingival plaque samples were collected for the detection and quantification of Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola, Micromonas micros, Prevotella intermedia, Fusobacterium genus and Streptococcus mutans by means of real-time PCR (qPCR). Patient's compliance, satisfaction and side effects were registered. Results Twenty-three patients in the essential oil group (mean age: 57) and 21 in the placebo group (mean age: 55) with acceptable oral hygiene at intake (mean PI <1.5 on a scale of 5) adhered to the study protocol. Gingivitis index, PI and BoP significantly reduced over time (P ≤ 0.029); however, between group analyses revealed no significant differences. There was no significant change over time neither in detection frequency nor load for any of the microbiota. Daily rinsing with an essential oil rinse was found safe and perceived beneficial by the patients. Conclusion Patients in supportive periodontal care who are fairly compliant with oral hygiene may not benefit from additional mouth rinsing using an essential oil solution.