The aim of this study is to assess whether and how a mouthwash containing an ingredient that reduces inflammation in combination with a recently developed treatment is more effective at improving advanced gum disease. Advanced gum disease, also called periodontitis, is an inflammation of the gums which results in decrease in bone levels supporting teeth and can result in tooth loss. Minimally invasive non-surgical treatment (MINST) is a type of deep cleaning of teeth which involves removal of plaque beneath the gum margin and reduces inflammation. In this trial we will assess the additional benefits of combining deep cleaning (MINST) with the use of a mouthwash known to have a beneficial effect on tissue healing by helping the body to 'switch off' inflammation in periodontitis. Participants may be part of a control group, where you they be treated by MINST and provided with a placebo mouthwash The placebo mouthwash does not contain the active ingredient which reduces inflammation. Neither the participants nor the investigators can decide or be aware of which mouthwash the participants be given, as this will be determined randomly and the two mouthwash packages look the same.

The duration of the study will be over six months and will consist of a total of seven appointments. The aim of this study is to assess whether we can achieve better outcomes using this mouthwash and the healing of thegums will be assessed clinically. In addition, other parameters such as saliva samples, blood samples and plaque samples will be collected throughout the study, which allows us to examine the healing process in more detail and will help us understand how exactly the combination of MINST and the mouthwash improve healing.

This is a pilot randomised controlled trial comparing patients being treated with MINST non-surgical therapy with an adjunctive test or placebo mouthwash. This will be a single centre trial in the Department of Periodontology at Guy's Dental Hospital. Patients will be recruited and treated at this centre as per the inclusion criteria. The primary outcome is PPD reduction at 6 months in test vs control group, and the secondary outcomes are CAL reduction at 6 months, changes in inflammatory/tissue degradation/repair biomarkers in saliva and GCF between baseline and 6 months post-treatment.

Inclusion Criteria:

1. Age 18 to 70
2. Diagnosis of periodontitis stage III-IV
3. Presence of at least 1 deep intrabony defect (both PPD and radiographic defects >6mm)
4. Full-mouth dichotomous plaque score>30%
5. Willing to voluntarily sign the informed consent.

Exclusion Criteria:

1. Previous periodontal treatment to the study site within 6 months
2. Use of systemic antibiotics within 3 months
3. Weekly regular use of non-steroidal anti-inflammatory drugs
4. Contributing medical history according to the examining clinician (e.g. diabetes mellitus)
5. Smoking history (current or within 5 years)
6. Allergic to any ingredient present in the products used in study.
7. Presence of systemic diseases, which affects bone and/or connective tissue metabolism
8. Concurrent participation in other clinical studies.
9. Pregnant or breastfeeding.