This is a randomized, double-blind clinical trial on the prophylactic use of antibiotics in postoperative endoscopic sinus vs placebo surgery. Patients with chronic rhinosinusitis with or without polyps who have consented to endoscopic sinus surgery according to Canadian practice guidelines may be included in the study after approval by the Research Ethics Board of the University Hospital Center. Quebec and signature of consent.

The severity of the disease will be evaluated preoperatively according to the SNOT-22 score, the visual analogue scale of nasal symptoms, the Lund-Mackay score on the CT-scan and the Modified Lund-Kennedy endoscopic score (sinusoscopy will be recorded). Relevant demographics and medical history of participants will also be collected preoperatively.

The extension of the CES and intraoperative findings will be noted. Patients who have an infection during the procedure (pus with positive culture) will be excluded from the study. During the surgery, 4 groups will be formed as described in the figure below. Patients will be divided according to the presence or absence of polyps at the endoscopy and randomized in one of the 2 groups by the pharmacy of the CHU of Quebec. This will provide either a prescription of Clavulin 875 mg per os twice daily for 10 days, or a placebo of similar visual appearance in the same dosage. A bioabsorbable dressing (NasoPore, Stryker) will be positioned at the mid-meatus level at the end of the procedure. Nasal irrigations of saline solution will be prescribed post-operatively (qid for 1 month) as well as intra-nasal corticosteroids bid after 1 week (usual treatment). The addition of systemic corticosteroids will be left to the judgment of the surgeon, noted and analyzed as a confounding factor.

Follow-up will be done at 2 weeks, 1 month, 3 months and 6 months post-surgery. Patients will have to complete the SNOT-22 quality of life score and the visual similar scale of nasal symptoms at each visit. Sinusoscopy will be recorded at scheduled visits postoperatively. A single blind evaluator (to limit inter-rater differences) will analyze the video recordings to establish the modified Lund-Kennedy endoscopic score for each patient. Middle-meat secretion culture will be performed if pus is present during sinusoscopy and an antibiotic prescribed if needed. The patient will have to fill in a diary of other medications (analgesics, narcotics, anti-inflammatories) as well as a diary of side effects that will be collected at the visit of a post-operative month.

Inclusion Criteria:

• 18 years old and over able to consent
• Patients with chronic rhinosinusitis and failure to maximal medical treatment
• Endoscopic sinus surgery with at least one of the following: maxillary antrostomy, ethmoidectomy, sphenoidotomy and / or frontal sinusotomy

Exclusion Criteria:

• Antibiotherapy less than 2 weeks before the intervention
• Penicillin allergy
• Inability to establish follow-up
• Open sinus surgery or associated septorhinoplasty (simple septoplasty is not considered an exclusion criterion)
• Immunodeficiency
• Cystic fibrosis of the pancreas
• Pregnancy
• odontogenic sinusitis
• Fungal sinusitis
• Diabetic
• Ciliary dyskinesia
• Sinus neoplasia
• Patient requiring antibiotic prophylaxis for endocarditis

Randomisation by the pharmacy, no one in the investigator and the participant will know the intervention