To compare the effect of N-acetylcysteine versus calcium hydroxide as intra canal medication on:

• Intensity of postoperative pain.
• Bacterial load reduction.
• levels of MMP -9 in Periapical Fluids.

Inclusion Criteria:

• Patients who are free from any physical or mental handicapping condition.

• Males & Females with

  • Mandibular single rooted permanent premolar teeth.
  • Absence of spontaneous pain.
  • Slight widening in the periodontal membrane space or with periapical radiolucency not exceeding 2*2 mm radiographically.

• Patients' acceptance to participate in the trial.

• Patients who can understand pain scale and can sign the informed consent (Appendix I)

Exclusion Criteria:

1. Medically compromised patients.
2. Pregnant women
3. Patients undergoing previous endodontic treatment or analgesics and/or antibiotics have been administrated by the patient during the past 24 hours preoperatively.
4. Patients reporting bruxism or clenching.
5. Teeth that show association with acute periapical abscess and swelling.
6. Greater than grade I mobility or pocket depth greater than 5mm.
7. Non-restorable teeth or teeth that could not be adequately isolated with a rubber dam
8. Teeth with vital pulp.
9. Immature teeth.
10. Radiographic evidence of external or internal root resorption vertical root fracture, perforation, calcification.