Abstract: We included 35 studies (including seven from the previous version of the review), which included 2565 participants in total. All studies used a parallel RCT design, and 33 studies (94%) only targeted T2DM patients. There was variation between studies with regards to included age groups (ages 18 to 80), duration of follow-up (3 to 12 months), use of antidiabetic therapy, and included participants' baseline HbA1c levels (from 5.5% to 13.1%). We assessed 29 studies (83%) as being at high risk of bias, two studies (6%) as being at low risk of bias, and four studies (11%) as unclear. Thirty-four of the studies provided data suitable for analysis under one or both of the two comparisons. Comparison 1: low quality evidence from 14 studies (1499 participants) comparing periodontal therapy with no active intervention/usual care demonstrated that mean HbA1c was 0.29% lower (95% confidence interval (CI) 0.48% to 0.10% lower) 3 to 4 months post-treatment, and 0.02% lower after 6 months (five studies, 826 participants; 95% CI 0.20% lower to 0.16% higher). Comparison 2: 21 studies (920 participants) compared different periodontal therapies with each other. There was only very low quality evidence for the multiple head-to-head comparisons, the majority of which were unsuitable to be pooled, and provided no clear evidence of a benefit for one periodontal intervention over another. We were able to pool the specific comparison between scaling and root planing (SRP) plus antimicrobial versus SRP and there was no consistent evidence that the addition of antimicrobials to SRP was of any benefit to delivering SRP alone (mean HbA1c 0.00% lower: 12 studies, 450 participants; 95% CI 0.22% lower to 0.22% higher) at 3-4 months post-treatment, or after 6 months (mean HbA1c 0.04% lower: five studies, 206 patients; 95% CI 0.41% lower to 0.32% higher). Less than half of the studies measured adverse effects. The evidence was insufficient to conclude whether any of the treatments were associated with harm. No other patient-reported outcomes (e.g. quality of life) were measured by the included studies, and neither were cost implications or diabetic complications. Studies showed varying degrees of success with regards to achieving periodontal health, with some showing high levels of residual inflammation following treatment. Statistically significant improvements were shown for all periodontal indices (BOP, CAL, GI, PI and PPD) at 3-4 and 6 months in comparison 1; however, this was less clear for individual comparisons within the broad category of comparison 2. There is low quality evidence that the treatment of periodontal disease by SRP does improve glycaemic control in people with diabetes, with a mean percentage reduction in HbA1c of 0.29% at 3-4 months; however, there is insufficient evidence to demonstrate that this is maintained after 4 months. There was no evidence to support that one periodontal therapy was more effective than another in improving glycaemic control in people with diabetes mellitus. In clinical practice, ongoing professional periodontal treatment will be required to maintain clinical improvements beyond 6 months. Further research is required to determine whether adjunctive drug therapies should be used with periodontal treatment. Future RCTs should evaluate this, provide longer follow-up periods, and consider the inclusion of a third 'no treatment' control arm. Larger, well conducted and clearly reported studies are needed in order to understand the potential of periodontal treatment to improve glycaemic control among people with diabetes mellitus. In addition, it will be important in future studies that the intervention is effective in reducing periodontal inflammation and maintaining it at lowered levels throughout the period of observation.

Plain-language summary: This review of existing clinical trials was carried out by authors working with the Cochrane Oral Health Group and updates the previous version published in 2010. The evidence is current up to 31 December 2014. In this review there are 35 trials (including 2565 participants), published between 1997 and 2014, where people randomly received a type of gum disease treatment (including scaling and root planing (SRP) and SRP combined with other types of treatment), or usual care/no active treatment. The trials included in this review used SRP with, or without, an additional treatment. Additional treatments included instructions for cleaning teeth properly (known as oral hygiene instruction (OHI)), and other gum treatments (for example, antimicrobials, which are used to treat infections). We found 35 trials that were suitable for inclusion in this review. Thirty-four of those studies provided results that could be included in at least one of the two comparisons. 1. The evidence from 14 trials (1499 participants) showed that SRP reduces blood sugar levels in diabetic patients by 0.29% up to 4 months after receiving care when compared with usual care/no active treatment. After 6 months, there was no evidence that this reduction was sustained. 2. The evidence from 21 trials (920 participants) investigating different types of gum disease treatments failed to show that one treatment was better than another. There were not enough studies measuring side effects to be able to show if gum disease treatments cause any harm. Currently there is low quality evidence to support using scaling and root planing for controlling blood sugar levels up to 4 months after receiving treatment. Ongoing gum disease treatment is advised to maintain improvements in blood sugar levels.