The goal of this clinical trial is to learn if vitamin C and N-acetylcysteine work to improve shoulder functional score after rotator cuff repair. It will also learn about the safety of vitamin C and N-acetylcysteine. The main questions it aims to answer are:

Do vitamin C and N-acetylcysteine improve the shoulder functional score of participants? Do vitamin C and N-acetylcysteine improve healing of rotator tendon? Researchers will compare vitamin C and N-acetylcysteine to a placebo (a look-alike substance that contains no drug) to see if vitamin C and N-acetylcysteine works to improve shoulder functional score after rotator cuff repair.

Participants will:

Take vitamin C and N-acetylcysteine or a placebo every day for 45 days. Visit the OPD three times at 2 week, 6 month and 12 month after surgery for checkups and tests.

Inclusion Criteria:

• All patient who undergone arthroscopic primary rotator cuff complete repair.
• Clinical history and symptom consistent with rotator cuff tear.
• MRI confirmation of rotator cuff tear.

Exclusion Criteria:

• Non-reparable or partial rotator cuff repair.
• High grade fatty degeneration of the rotator cuff (Goutallier III, IV)
• At least one torn tendon with retraction to the glenoid (stage 3 of Patte classification)
• Rotator cuff retear.
• Isolated subscapularis tear.
• Patient who loss follow up.
• Patent who cannot respond questionnaire.
• Patient who allergy to vitamin C or N-acetylcysteine.