The aim of this prospective, randomized, placebo controlled study is to evaluate the effect of amoxicillin/clavulanic acid combination on post-operative pain and swelling in adult patient with symptomatic apical periodontitis.

Medical and dental history will be obtained from all patients participating in this trial. Clinical and radiographic evaluation for each tooth included in this study will be recorded.

Clinical diagnosis of symptomatic apical periodontitis is to be confirmed.

After single-visit root canal treatment, eligible participants will be randomly assigned to one of the following groups: experimental group (taking oral tablet 1000 mg of Amoxicillin/ clavulanic acid combination) or control group (taking placebo tablet).

Each patient will receive a 7-day diary to record postoperative pain and swelling.In case of pain, the participant will be instructed to take an analgesic. Post-operative pain will be measured as a primary outcome on 11-point scale (NRS) at the following time points: 6, 12, 24, 48, 72 hours and 7 days. The occurrence of swelling will be reported by participant as a secondary outcome.

Inclusion Criteria:

1. Patients with symptomatic apical periodontitis who do not have evidence of spreading infection or systemic involvement.
2. Mandibular posterior teeth with positive response to percussion.
3. Patients with non-contributory systemic condition.
4. Patients who can understand pain scales and able to sign informed consent.

Exclusion Criteria:

1. Patients allergic to penicillin.
2. Patients who have a draining sinus tract.
3. Retreatment cases.