Therapeutic Dosing of Cloxacillin for MSSA Impetigo
For impetigo caused by methicillin-susceptible Staphylococcus aureus (MSSA), cloxacillin or its analog dicloxacillin should be dosed at 250-500 mg orally four times daily in adults (or 25 mg/kg/day divided into four doses in children) for 7 days. 1
Guideline-Based Antibiotic Selection
The IDSA guidelines specifically recommend dicloxacillin (cloxacillin's close analog) or cephalexin as first-line agents for impetigo when MSSA is suspected or confirmed, given that most S. aureus isolates from impetigo are methicillin-susceptible. 1 The treatment duration should be 7 days for uncomplicated impetigo. 1
Standard Dosing Regimens
Oral Therapy (Outpatient)
- Adults: Cloxacillin 250-500 mg orally four times daily (every 6 hours) 2, 3
- Children: 25 mg/kg/day divided into four doses 4
- Duration: 7 days for impetigo 1
Intravenous Therapy (Severe Infections)
- Adults: 2 g intravenous every 4 hours (12 g/day total) achieves optimal pharmacodynamic targets with time above MIC >50% 2
- Alternative dosing: 3 g intravenous every 6 hours (four times daily) maintains time above MIC at 42% 2
Pharmacodynamic Rationale
Cloxacillin is a time-dependent bactericidal antibiotic requiring the drug concentration to remain above the minimum inhibitory concentration (MIC) for at least 40-50% of the dosing interval to achieve optimal effectiveness. 2 The standard dosing of 2 g every 4 hours intravenously or divided oral dosing four times daily achieves this pharmacodynamic target. 2
Clinical Context for MSSA Impetigo
While the IDSA guidelines mention dicloxacillin specifically, cloxacillin is pharmacologically equivalent and widely used internationally as a penicillinase-resistant penicillin. 1, 3 Both agents remain the antibiotics of choice for serious MSSA infections due to their bactericidal activity, tissue distribution, and safety profile. 2, 3
Important Caveats
Avoid if MRSA suspected: Beta-lactams including cloxacillin are completely ineffective against methicillin-resistant strains and should never be used when MRSA is suspected. 5, 6 Switch to TMP-SMX, clindamycin, or doxycycline if MRSA is confirmed. 1
Narrow spectrum advantage: Cloxacillin's narrow microbiological spectrum targeting staphylococci reduces selection pressure for resistance compared to broader-spectrum agents. 2
Streptococcal coverage: If cultures yield streptococci alone, oral penicillin (not cloxacillin) becomes the preferred agent. 1
Take on empty stomach: Oral absorption is reduced by food; administer 1 hour before or 2 hours after meals for optimal bioavailability. 3