The goal of this clinical trial is to learn if antibiotics can prevent the formation of fistula-in-ano after incision and drainage of a first time of perianal abscess in adults. A perianal abscess is a collection of infected fluid near the anus. A Fistula-in-ano is an abnormal tunnel that can form between the anal canal and the skin after an abscess.

The main questions it aims to answer are:

Does taking antibiotics after incision and drainage of a perianal abscess lower the chance of developing a Fistula-in-ano? What medical problems or side effects do participants have after taking antibiotics or not taking antibiotics?

Researchers will compare participants who recieve antibiotics after abscess drainage with paticipants who do not recieve antibiotics after abscess drainage, to see if antibiotics lower the chance of developing a Fistula-in-ano.

Participants will:

Be randomly assigned to recieve antibiotics for 7 days after abscess incision and drainage or to not recieve antibiotics after abscess incision and drainage.

Attend follow-up clinic visits after incision and drainage as part of their standard of care.

Complete follow-up phone call about 12 months after incision and drainage. Allow the study team to review their medical record for abscess treatment, symptoms, recurrence, fistula-in-ano formation, additional procedures, and antibiotics side effects.

This is a single-center, randomized, open-label clinical trial evaluating whether a 7-day course of postoperative antibiotics after standard of care incision and drainage of a first-time perianal abscess reduces subsequent anal fistula formation compared with no postoperative antibiotics.

Perianal abscess is a common anorectal condition treated primarily with incision and drainage. After drainage, some patients later develop an anal fistula, which is an abnormal tract between the anal canal and the perianal skin. The role of routine antibiotics after adequate drainage remains uncertain, especially in patients without high-risk clinical features that would otherwise require antibiotics as part of standard care. This study is designed to evaluate whether a short postoperative antibiotic course can reduce fistula formation after a first episode of perianal abscess.

Eligible adult participants with a first-time perianal abscess requiring incision and drainage will be randomized in a 1:1 ratio to either postoperative antibiotics or no postoperative antibiotics. Participants assigned to the antibiotic arm will receive amoxicillin/clavulanate for 7 days. Participants with penicillin allergy may receive ciprofloxacin plus metronidazole instead. Participants assigned to the no-antibiotic arm will not receive postoperative antibiotics unless antibiotics are later determined to be medically necessary by the treating clinician.

All participants will receive standard clinical care for the perianal abscess, including incision and drainage as determined by the treating surgical team. The study does not change the technical approach to drainage or other clinically indicated care. The research intervention is the randomized postoperative antibiotic strategy.

Participants will be followed through routine clinical visits and a 12-month follow-up phone call. The study will collect information from the medical record and follow-up contacts regarding fistula formation, recurrent abscess, additional procedures or treatment, emergency department visits or readmissions, antibiotic use, medication adherence, and antibiotic-related adverse events. The findings may help clarify whether routine postoperative antibiotics should be used after drainage of first-time perianal abscess in patients without other clinical indications for antibiotics.

Inclusion Criteria:

• Age 18 years or older
• First episode of perianal abscess requiring incision and drainage
• Able to provide informed consent
• English or Spanish speaking patient
• Treating surgical team has determined that incision and drainage is clinically indicated as part of standard care

Exclusion Criteria:

• Known fistula-in-ano at presentation
• Prior perianal abscess within 12 months
• History of inflammatory bowel disease, including Crohn's disease or ulcerative colitis
• Known or suspected anorectal malignancy
• Necrotizing soft tissue infection or Fournier's gangrene
• Systemic infection or sepsis
• Significant surrounding cellulitis or extensive soft-tissue infection for which antibiotics are clinically indicated
• Immunocompromised or immunosuppressed status, including active chemotherapy or radiation therapy, history of transplant, chronic systemic steroids, biologic immunosuppressive therapy, advanced or uncontrolled HIV/AIDS, or other clinically significant immunosuppression
• Cardiac condition requiring antibiotic prophylaxis or antibiotic therapy for procedures involving infected tissue
• Need for non-study antibiotics at the time of enrollment for another infection or medical indication
• Contraindication, allergy, or intolerance to all study antibiotic regimens
• Pregnant
• Prisoner
• Minor
• Adult unable to provide informed consent

Individual participant data will not be made publicly available. Aggregate study results will be reported as required and may be published in scientific presentations or manuscripts.

Open-label trial. Participants and treating clinicians will know the assigned postoperative management strategy. Outcomes will be assessed through clinical follow-up and medical record review.