What is the dose of Keflex (cephalexin) for Staphylococcus aureus infections?

Cephalexin Dosage for Staphylococcus aureus Infections

For Staphylococcus aureus infections, the recommended dose of cephalexin is 500 mg orally four times daily for adults, and 25-50 mg/kg/day divided into four doses for children. 1

Adult Dosing

Methicillin-Susceptible Staphylococcus aureus (MSSA)

• Standard dosing: 500 mg orally four times daily 1
• Alternative dosing: 500 mg every 6 hours 2
• Duration of therapy: 7 days for uncomplicated skin infections, depending on clinical response 2

Special Considerations

• For more severe infections or those caused by less susceptible organisms, larger doses may be needed 1
• Daily doses should not exceed 4 g; if higher doses are required, parenteral cephalosporins should be considered 1
• Alternative twice-daily dosing (500 mg every 12 hours) may be considered for improved adherence, though this is not the FDA-approved regimen 3, 4

Pediatric Dosing

Standard Dosing

• 25-50 mg/kg/day divided into four doses 1
• For skin and skin structure infections, the total daily dose may be divided and administered every 12 hours 1

Weight-Based Dosing Guidelines

For cephalexin suspension:

• 10 kg (22 lb): 1/2 to 1 tsp (125 mg/5 mL) four times daily or 1/4 to 1/2 tsp (250 mg/5 mL) four times daily 1
• 20 kg (44 lb): 1 to 2 tsp (125 mg/5 mL) four times daily or 1/2 to 1 tsp (250 mg/5 mL) four times daily 1
• 40 kg (88 lb): 2 to 4 tsp (125 mg/5 mL) four times daily or 1 to 2 tsp (250 mg/5 mL) four times daily 1

Special Considerations

• In severe infections, the dosage may be doubled 1
• For otitis media, 75-100 mg/kg/day in 4 divided doses is required 1
• Recent research suggests that twice-daily (BID) dosing of 22-45 mg/kg may be effective for MSSA with MICs of 1-2 mg/L, and 80 mg/kg for MICs of 4 mg/L 5
• Thrice-daily (TID) dosing of 15-25 mg/kg may be effective for MSSA with MICs of 1-2 mg/L, and 45 mg/kg for MICs of 4 mg/L 5

Important Clinical Considerations

MRSA Coverage

• Cephalexin is NOT effective against methicillin-resistant Staphylococcus aureus (MRSA) 2
• For suspected or confirmed MRSA infections, alternative agents should be used:
  • Vancomycin (IV) 2
  • Clindamycin (if susceptible) 2
  • Trimethoprim-sulfamethoxazole 2
  • Linezolid 2
  • Doxycycline (for patients >8 years old) 2

Duration of Therapy

• For uncomplicated skin infections: 7 days 2
• For β-hemolytic streptococcal infections: at least 10 days 1
• For more complicated infections (osteomyelitis, septic arthritis), longer durations are required 2

Monitoring and Follow-up

• Clinical response should be evident within the first week of therapy 4
• If no improvement is seen within 48-72 hours, consider:
  • Drainage of any undrained collections 2
  • Obtaining cultures to rule out MRSA 2
  • Switching to an alternative antimicrobial agent 2

Caveats and Pitfalls

• Always consider local resistance patterns when prescribing cephalexin for S. aureus infections 2
• Cephalexin is ineffective against MRSA, which is increasingly common in community settings 6
• For abscesses, incision and drainage is the primary treatment; antibiotics may be unnecessary for uncomplicated cases after adequate drainage 6
• Patients with penicillin allergy may still tolerate cephalexin, but those with immediate hypersensitivity reactions should avoid it 2
• Dose adjustment is required in patients with severe renal impairment (creatinine clearance <30 mL/min) 7

Remember that cephalexin is a first-generation cephalosporin that remains effective against MSSA but should not be used empirically when MRSA is suspected or in areas with high MRSA prevalence without confirming susceptibility.