Why is cefalexin (a cephalosporin antibiotic) used to treat an infected sebaceous cyst in an adult patient?

Why Cefalexin is Used for Infected Sebaceous Cysts

Cefalexin is used for infected sebaceous cysts because it provides effective oral coverage against Staphylococcus aureus and Streptococcus pyogenes, the predominant pathogens in skin and soft tissue infections, and is FDA-approved specifically for this indication. 1

Microbiological Rationale

The choice of cefalexin is based on the typical bacterial flora involved in infected epidermoid (sebaceous) cysts:

• Staphylococcus aureus is the single most common pathogen in cutaneous abscesses and infected cysts, present in approximately 25% of cases as a sole organism 2
• Cefalexin has documented antimicrobial activity against both S. aureus and S. pyogenes, the two most frequent pathogens in skin and soft tissue infections 1, 3
• The FDA label explicitly lists "skin and skin structure infections caused by Staphylococcus aureus and/or Streptococcus pyogenes" as an approved indication 1

Important Clinical Context About Infected Cysts

A critical caveat: most inflamed epidermoid cysts are NOT truly infected. The inflammation typically occurs as a sterile reaction to rupture of the cyst wall and extrusion of keratinous contents into the dermis, rather than bacterial infection 2. Even uninflamed cysts contain normal skin flora in their contents 2.

When Antibiotics Are Actually Indicated

Systemic antibiotics like cefalexin are rarely necessary for most infected cysts 2. Antibiotics should be reserved for specific circumstances:

• Multiple lesions present 2
• Extensive surrounding cellulitis beyond the immediate cyst area 2
• Severe systemic manifestations such as high fever 2
• Severely impaired host defenses (immunocompromised patients) 2
• Cutaneous gangrene or necrotizing features 2, 4

Primary Treatment Approach

The definitive treatment for infected cysts is surgical, not medical:

• Incision and drainage with thorough evacuation of pus is the cornerstone of management 2
• Probing the cavity to break up loculations ensures complete drainage 2
• Simply covering with a dry dressing is usually sufficient post-drainage 2
• Gram stain and culture are rarely necessary unless systemic antibiotics are being considered 2

Pharmacological Advantages of Cefalexin

When antibiotics are deemed necessary, cefalexin offers specific practical benefits:

• Complete and rapid absorption from the upper intestine, achieving therapeutic tissue levels 3
• High oral bioavailability without gastrointestinal irritation, even at relatively high doses 3
• Does not disturb lower bowel flora since absorption occurs high in the intestinal tract 3
• Low incidence of allergic reactions due to its chemical stability and configuration 3
• Excellent urinary and tissue concentrations that exceed minimum inhibitory concentrations for typical pathogens 3

Critical Limitations and Warnings

Cefalexin has NO activity against Pseudomonas aeruginosa 5. This is not typically relevant for sebaceous cysts but becomes critical in other skin infections involving cartilage or specific trauma patterns 5.

Risk of C. difficile infection exists with cefalexin use:

• Cephalosporins are considered high-risk antibiotics for C. difficile infection by the CDC 6
• Risk persists during treatment and up to 2 months after cessation, with highest risk in the first month (7-10 fold increase) 6
• Discontinue immediately if C. difficile infection is suspected 6

Clinical Algorithm for Management

For uncomplicated infected sebaceous cyst:

1. Perform incision and drainage as primary treatment 2
2. Apply dry dressing 2
3. Reserve antibiotics only if extensive cellulitis, fever, or systemic signs present 2

If antibiotics are indicated:

1. Cefalexin 500mg orally four times daily for 7-10 days (typical dosing for skin infections) 1, 3
2. Adjust dose if creatinine clearance <30 mL/min 3
3. Monitor for C. difficile symptoms during and up to 2 months after treatment 6

Avoid cefalexin if:

• History of anaphylaxis, angioedema, or urticaria to penicillins (cross-reactivity risk) 2
• Suspected MRSA infection (requires alternative coverage) 2