What is the appropriate cephalexin (Keflex) dosage for an uncomplicated furuncle in adults and children, and how should the dose be adjusted for severe penicillin or cephalosporin allergy, suspected MRSA infection, or impaired renal function?

Cephalexin Dosing for Boils (Furuncles)

Standard Dosing Regimen

For uncomplicated boils in adults, cephalexin 500 mg orally four times daily (every 6 hours) is the recommended dose, typically for 7-10 days. 1, 2

Adult Dosing

• 500 mg four times daily (every 6 hours) is the standard dose for skin and soft tissue infections caused by methicillin-susceptible Staphylococcus aureus (MSSA), the most common pathogen in boils 1, 2
• For mild infections, 250 mg four times daily may be considered, though 500 mg is preferred for adequate tissue penetration 1
• Treatment duration is typically 7-10 days depending on clinical response 2

Pediatric Dosing

• 25-50 mg/kg/day divided into 4 doses for mild to moderate skin infections 1, 2
• For confirmed MSSA infections requiring more aggressive therapy, increase to 75-100 mg/kg/day divided into 3-4 doses 2
• The availability of suspension formulation makes cephalexin practical for pediatric use 1

When Cephalexin Is NOT Appropriate

MRSA Suspected or Confirmed

If the patient has failed initial antibiotic treatment, has systemic signs of infection (fever, tachycardia, hypotension), or MRSA is suspected, do NOT use cephalexin. 1

Switch to MRSA-active agents:

• Trimethoprim-sulfamethoxazole 1-2 double-strength tablets twice daily (adults) or 8-12 mg/kg/day based on trimethoprim component in 2 divided doses (children) 1
• Clindamycin 300-450 mg four times daily (adults) or 30-40 mg/kg/day in 3 divided doses (children) 1
• Doxycycline 100 mg twice daily (adults only, not for children <8 years) 1

Severe Penicillin Allergy

• Cephalexin can be used in patients with non-severe penicillin allergy (e.g., delayed rash) 1, 2
• Contraindicated in patients with history of anaphylaxis, angioedema, or urticaria to penicillins due to cross-reactivity risk 2
• For severe penicillin allergy, use clindamycin 300-400 mg four times daily instead 1, 3

Renal Dose Adjustment

Patients with creatinine clearance <30 mL/min require dose reduction proportional to their reduced renal function. 4

• Cephalexin is 70-100% renally excreted unchanged within 6-8 hours 4
• Dose adjustment should be based on creatinine clearance or serum creatinine levels 4
• Consider extending dosing intervals or reducing individual doses in significant renal impairment 4

Critical Clinical Considerations

Incision and Drainage Is Primary

• Antibiotics are adjunctive to incision and drainage for boils/abscesses 1
• Systemic antibiotics are indicated when there are signs of systemic inflammatory response syndrome (SIRS): temperature >38°C or <36°C, heart rate >90 bpm, respiratory rate >24/min, or WBC >12,000 or <4,000 cells/μL 1

Limitations of Cephalexin

• Completely ineffective against MRSA - do not use if MRSA is suspected 2
• Poor activity against Haemophilus influenzae and Moraxella catarrhalis 2
• Limited anaerobic coverage - not suitable for deep abscesses with anaerobic involvement 2

Adherence Strategy

• Four-times-daily dosing can be challenging for adherence 5
• Specific timing (e.g., 6 AM, 12 PM, 6 PM, 12 AM) helps maintain consistent drug levels 2
• For pediatric patients with compliance concerns, recent evidence suggests twice or thrice-daily dosing may be explored, though four-times-daily remains the guideline standard 5