Objective: To evaluate the effectiveness of systemic antibiotic prophylaxis 2 g amoxicillin/125 clavulanic acid in preventing infection after extracting inferior third molars totally covered by bone in healthy adults.

Methods: A double-blind placebo-controlled randomized clinical trial (EUDRACT 2008-005663-34). The sample was derived from the population of subjects attending Cruces Hospital for extraction of an inferior third molar totally covered by bone under loco-regional anaesthesia. Patients were treated with preoperative placebo or amoxicillin/clavulanic acid 2000/125 mg, 1 h before the surgery and post-operatively twice a day for 4 days. 0.2% chlorhexidine gluconate rinses were used. The outcome variable was infectious and inflammatory complications. Sex, age, smoking, molar depth, angulation, C-reactive protein and operation time were recorded. Analysis was by intention to treat.

Results: In 33 subjects (16 treated with antibiotic and 17 with placebo), the frequency of infection was 6.25% in the antibiotic group and 11.76% in the placebo group (OR 0.5, 95%CI (0.04–6.12). The number needed to treat was 18 (4, infinity). Absolute risk reduction ARR 5.5% of patients will not be infected under antibiotic therapy that they would have under control, 95% confidence interval (−13.86–24.89%) crosses zero which implies the ARR is not significant.

Conclusion: Amoxicillin/clavulanic acid 2 g/125 reduces the incidence of infection following inferior third molar extraction when the molar is totally covered by bone however this reduction is not statistically significative and we consider antibiotic should not be prescribed. Supported by the Health research fund, Carlos III Health Institute.

Conflict of interest: None declared.

Funding: This work was supported by the FIS GRANT EC08/00068 .