The aim of this prospective, randomized, placebo controlled study is to evaluate the effect of clindamycin 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.

Before root canal treatment, eligible participants will be randomly assigned to one of the following groups: experimental group (taking 600 mg of clindamycin orally) or control group (taking placebo ) 30 minutes before the start of the treatment. Single-visit root canal treatment will be performed.

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 and do not have evidence of spreading infection or systemic involvement.
2. Mandibular posterior teeth.
3. Patients in good health.
4. Patients who can understand pain scales (NRS).
5. Patients able to sign informed consent.

Exclusion Criteria:

1. Patients who have draining sinus tract.
2. Retreatment cases
3. Patients with weeping canals.

Double Blind (Subject, Caregiver, Investigator, Outcomes Assessor) Primary Purpose: Treatment