Purpose: This randomized study assesses the effects of antibiotics on the formation of fistulae after drainage of anorectal abscesses.

Methods: Patients who underwent abscess drainage in 3 major colorectal units between September 2005 and January 2008 were included. Previous anorectal surgery history, immunecompromised states, pregnancy, inflammatory bowel disease, antibiotic usage prior to surgery and the presence of an anal fistulae at the time of surgery were the exclusion criteria. Patients were randomized and given either placebo or amoxicillin-clavulanic acid combination treatment for 10 days. Patients were followed one year for perianal fistulae formation.

Inclusion Criteria:

• Presence of anorectal abscess

Exclusion Criteria:

• Allergy to penicillin derivatives
• Treatment with any antimicrobial agent in the week before enrolment
• Recognized other infection at the time of surgery
• Previous anorectal surgery
• Presence of inflammatory bowel disease
• Suspicion of Fournier's gangrene
• Secondary and recurrent anorectal abscesses
• Presence of an internal opening
• Any additional surgical procedure performed in the same session or during follow-up
• Antibiotic prophylaxis indicated for another reason
• Immunosuppressive and/or anticoagulant drug treatment being taken at the time of surgery
• Diabetes mellitus
• Known chronic disabling disease
• Pregnancy
• Lactation.