Mouthwashes or rinses have been used for centuries, but it is only in the last 50–60 years that carefully formulated and tested commercial products have become widely available. The aim of this review was to assesses the effectiveness of one group of mouthwashes, those containing essential oils, in reducing plaque levels when used as an adjunct to mechanical plaque control.

The authors have searched a good range of databases with no restrictions on language or date of publication. Good methodological approaches were used for the selection, data abstraction and quality assessment of the included studies; only studies with a minimum of six months' duration being included. Plaque and gingivitis scores were the main outcomes considered. One hundred and seven potentially relevant studies were identified with a large proportion (39) being excluded because they were shorter than six months in duration. All the included studies were industry funded and testing Listerine® products. Assessment of study quality suggests that around half of the studies were at low risk for random sequence generation but there was less clarity on allocation concealment. However, only one of the included studies was unclear regarding blinding of the outcome assessors. This led the authors to conclude that the quality of the overall body of evidence was moderate to low.

All the included studies showed a statistically significant reduction in plaque for EO mouthwashes compared to placebo. A number of meta-analyses were conducted, for the Quigly-Hine Index the weighted mean difference (WMD) in favour of EO mouthwashes = −0.86 (95%CI; −1.05 to −0.66). EO mouthwashes were also more effective than placebo for all the gingival indices. This equates to a 32% reduction in plaque and a 24% improvement in modified gingival index. The meta-analyses demonstrated a high degree of heterogeneity and the authors investigated this using meta-regressions which suggest that much of the heterogeneity in the plaque scores could be explained by the percentage of males in the studies and whether the mouthwash use was supervised. For the gingival indices the provision of oral hygiene was a major contributor.

A 2015 meta-analysis by Araujo et al.1 also investigated the effect of EO mouthwashes on plaque and gingivitis. That review included 29 industry sponsored trials of six months or longer, 11 of which were unpublished. They calculated summary percentage reductions in whole-mouth mean plaque of 27.7% (95%CI; 22.4–32.9) and gingivitis of 16.0% (95%CI; 11.3–20.7) respectively. Four studies involving a total of 982 patients compared EO and CPC mouthwash with analysis indicating lower levels of plaque and gingivitis with EO mouthwashes.

While the Araujo review and this new review demonstrate a statistically significant improvement in plaque and gingivitis scores, the clinical importance of the findings is more difficult to assess. The review does not consider adverse effects. Most of the included studies are assessed against placebo so more head-to-head studies against other mouthwashes and other agents to improve oral heath are needed. For example a 2012 meta-analysis of chlorhexidine mouthrinse2 found that its use reduced plaque by 33% and gingivitis by 26%, and a 2013 Cochrane review by Riley and Lamont3 demonstrated a 42% reduction in plaque severity and 48% reduction in bleeding with triclosan/copolymer toothpaste use. However, the limited direct comparisons, both in terms of effectiveness and cost-effectiveness and knowledge of any potential adverse effects, make it difficult to recommend the most appropriate regimens for patients.