The primary objective of this controlled clinical trial is to evaluate the clinical effect of systemic amoxicillin plus metronidazole therapy in conjunction with initial non-surgical treatment of peri-implantitis. The secondary objective is to assess the microbiological effectiveness of this peri-implantitis treatment approach.

The present study is a single-blind, randomized controlled clinical trial. Adult patients with at least one endosseous implant in the oral cavity with clinical and radiographical evidence of periimplantitis will be included in this study.

Patients with implants affected by peri-implantitis will receive initial periodontal treatment existing of oral hygiene instructions and mechanical cleansing of both implants and remaining dentition. Immediately after initial periodontal therapy patients will be instructed to rinse their mouth with 0.12% chlorhexidine + 0.05% cetylpyridinium chloride without alcohol twice daily during 30 seconds for 2 weeks. Additionally patients will receive a recipe for medication consisting of 500 mg amoxicillin and 500 mg metronidazole to be taken every 8 hours for the following 7 days or no recipe.

The main study parameter is the mean peri-implant bleeding score.

Inclusion Criteria:

• The patient is ≥ 18 years of age;
• The patient has at least one endosseous implant in the oral cavity with clinical and radiographical signs of peri-implantitis. Peri-implantitis is defined as a loss of marginal bone ≥ 2 mm as compared to the shoulder of the implant (the level at which the bone is normally located immediately after implant placement), in combination with bleeding and/or suppuration on probing and a peri-implant probing depth ≥ 5 mm;
• The implants have been in function for at least two years;
• The patient is capable of understanding and giving informed consent.

Exclusion Criteria:

• Medical and general contraindications for the surgical procedures;
• A history of local radiotherapy to the head and neck region;
• Pregnancy and lactation;
• Uncontrolled diabetes (HbA1c < 7% or < 53 mmol/mol)
• Mononucleosis infectiosa
• Organic neurological disorders
• Use of antibiotics during the last 3 months;
• Known allergy to amoxicillin, metronidazole or chlorhexidine;
• Long-term use of anti-inflammatory drugs;
• Full edentulism (no remaining teeth, only implants)
• Incapability of performing basal oral hygiene measures as a result of physical or mental disorders;
• Implants placed in areas augmented with autogenous bone from the crista iliac region;
• Implants placed in skin grafted areas;
• Implants with bone loss exceeding 2/3 of the length of the implant or implants with bone loss beyond the transverse openings in hollow implants;
• Implant mobility;
• Implants at which no position can be identified where proper probing measurements can be performed;
• Previous surgical treatment of the peri-implantitis lesions;
• Previous non-surgical treatment of the peri-implantitis lesions during the last 6 months (scaling or curettage)