What is Oral Oxacillin?
Oral oxacillin is a penicillinase-resistant penicillin antibiotic indicated specifically for treating infections caused by penicillinase-producing staphylococci, but it is important to note that oxacillin is primarily administered intravenously rather than orally in modern clinical practice.
Indications
Oxacillin is indicated for infections caused by penicillinase-producing staphylococci that have demonstrated susceptibility to the drug 1. Key clinical uses include:
- Methicillin-susceptible Staphylococcus aureus (MSSA) infections, including:
Oxacillin should NOT be used for infections caused by organisms susceptible to penicillin G or for methicillin-resistant Staphylococcus aureus (MRSA) 1.
Dosing
Adults
- Mild to moderate infections: 250-500 mg IV every 4-6 hours 1
- Severe infections: 1 gram IV every 4-6 hours 1
- Prosthetic valve endocarditis: 12 g/24 hours IV divided into 6 equally divided doses (2 g every 4 hours) for ≥6 weeks 3
Pediatric Patients (<40 kg)
- Mild to moderate infections: 50 mg/kg/day IV in equally divided doses every 6 hours 1
- Severe infections: 100 mg/kg/day IV in equally divided doses every 4-6 hours 1
- Premature infants and neonates: 25 mg/kg/day IV 1
Duration of Therapy
- Minimum: Continue for at least 48 hours after the patient becomes afebrile, asymptomatic, and cultures are negative 1
- Severe staphylococcal infections: At least 14 days 1
- Endocarditis and osteomyelitis: Longer duration required 1
Important Precautions and Caveats
Allergy Considerations
Do not administer oxacillin to patients with a history of penicillin hypersensitivity 1. For patients with:
- Immediate-type hypersensitivity reactions to β-lactams: Use vancomycin instead 3
- Non-immediate-type hypersensitivity reactions: Cefazolin may be substituted 3
Monitoring Requirements
During prolonged therapy, perform periodic assessment of 1:
- Renal function (urinalysis, BUN, creatinine)
- Hepatic function (AST, ALT)
- Hematopoietic function (CBC with differential, blood cultures)
- Obtain these at least weekly during therapy
Drug Interactions
- Avoid concurrent tetracycline use: Tetracycline may antagonize oxacillin's bactericidal effect 1
- Probenecid: Increases and prolongs oxacillin blood levels by blocking renal tubular secretion; reserve this combination only for infections requiring very high serum levels 1
Administration Considerations
- Thrombophlebitis risk: Particularly in elderly patients with IV administration 1
- Tissue necrosis: Extravasation can cause tissue damage, especially at concentrations ≥50 mg/mL; avoid infusion pump administration in high-risk patients 4
Clinical Context
Oxacillin remains the parenteral drug of choice for MSSA infections 2, with excellent susceptibility patterns showing 99.2% sensitivity in MSSA isolates nationally 5. However, it has zero activity against MRSA 2, 5 and limited activity against coagulase-negative staphylococci (44.5% susceptibility) 5.
For prosthetic valve endocarditis caused by MSSA, oxacillin should be combined with rifampin (900 mg/24 hours in 3 divided doses for ≥6 weeks) plus gentamicin (3 mg/kg/24 hours for the initial 2 weeks) 3.
Always obtain cultures and susceptibility testing before initiating therapy, though oxacillin may be started empirically in suspected resistant staphylococcal infections while awaiting results 1.