What should I do about a large, uncomfortable boil under my left axilla that has not responded to amoxicillin (amoxicillin) treatment and has grown larger over 4 days?

Management of an Enlarging Axillary Boil Unresponsive to Amoxicillin

You need to stop the amoxicillin immediately and switch to either amoxicillin-clavulanate or a first-generation cephalosporin, but more importantly, this boil requires incision and drainage as the primary treatment—antibiotics alone will not resolve an established abscess. 1

Why Amoxicillin is Failing

Amoxicillin lacks coverage for the polymicrobial flora typical of axillary abscesses, which include both aerobic and anaerobic bacteria from skin and adjacent areas. 1 The axillary location specifically harbors mixed gram-negative organisms and anaerobes that require broader spectrum coverage than amoxicillin provides. 2

Immediate Management Steps

1. Incision and Drainage (Primary Treatment)

• The most important intervention is incision and drainage—this is mandatory and should not be delayed. 1
• All cutaneous abscesses require drainage regardless of size, as antibiotics alone cannot adequately treat an established abscess. 1
• The drainage procedure should include thorough evacuation of pus and probing the cavity to break up any loculations. 1
• For larger abscesses (>5 cm), multiple counter incisions may be needed rather than one long incision. 1

2. Antibiotic Selection for Axillary Location

For axillary abscesses specifically, the recommended antibiotics are:

• First choice: Amoxicillin-clavulanate (augmentin) 875 mg twice daily 2, 1
• Alternative: Cefoxitin or ampicillin-sulbactam 2
• These agents provide coverage for the mixed aerobic and anaerobic flora characteristic of axillary infections. 2, 1

3. When Antibiotics Are Indicated After Drainage

Antibiotics are necessary in your case because: 1

• The abscess has grown larger despite antibiotic treatment (suggesting systemic involvement)
• You describe significant discomfort (suggesting surrounding cellulitis)
• The infection has persisted for 4 days with worsening

Antibiotics should be given for 4-7 days based on clinical response. 1

Red Flags Requiring Urgent Evaluation

Seek immediate medical attention if you develop: 2

• Fever >38.5°C (101.3°F)
• Heart rate >100 beats/minute
• Spreading redness extending >5 cm from the boil
• Red streaking up the arm (lymphangitis)
• Severe pain or inability to move the arm

Common Pitfalls to Avoid

• Do not continue amoxicillin alone—it lacks the necessary coverage for axillary infections and will continue to fail. 2
• Do not attempt needle aspiration—this has a very low success rate of only 25% and is not recommended. 1
• Do not delay drainage while waiting for antibiotics to work—drainage is the definitive treatment and antibiotics are only adjunctive. 1
• Do not assume this is a simple infection—the fact that it's enlarging on antibiotics suggests either inadequate coverage or the need for source control through drainage. 1

Follow-Up Criteria

After drainage and appropriate antibiotics, you should see improvement within 48-72 hours. 1 If signs of infection persist beyond 7 days, you need diagnostic re-evaluation including possible imaging and culture of the drained material. 1