Oxacillin
Oxacillin is a narrow-spectrum, **beta-lactamase-resistant (anti-staphylococcal) penicillin**. * **Clinical Utility**: It is primarily indicated for treating bone, skin, and soft tissue infections caused by penicillinase-producing *Staphylococcus* species (e.g., MSSA, MSSP). * **Spectrum**: Highly effective against gram-positive cocci. It is inactive against *Rickettsia*, mycobacteria, fungi, *Mycoplasma*, viruses, and most gram-negative organisms. * **Resistance Context**: While it resists staphylococcal beta-lactamase, it is ineffective against **Methicillin-Resistant *Staphylococcus aureus* (MRSA)** or **Methicillin-Resistant *Staphylococcus pseudintermedius* (MRSP)**, which utilize a different resistance mechanism (altered target site via the *mecA* gene). * **Veterinary Limitations**: Its use in veterinary medicine is currently infrequent due to its very short half-life (requiring q6-8h dosing), poor oral bioavailability, and the lack of convenient veterinary-specific oral dosage forms. Cephalosporins or amoxicillin-clavulanate are often preferred for convenience.
Mechanism: Oxacillin is a **bactericidal beta-lactam antibiotic**. * **Mechanism**: It covalently binds to **Penicillin-Binding Proteins (PBPs)** (specifically transpeptidases) located on the inner membrane of the bacterial cell wall. * **Pathway**: PBP binding → inhibition of the terminal transpeptidation step of peptidoglycan synthesis → weakened cell wall → osmotic instability → **cell lysis and death**. * **Enzyme Resistance**: The drug features a bulky isoxazolyl side chain that provides steric hindrance, preventing staphylococcal **beta-lactamase (penicillinase)** from hydrolyzing and destroying the beta-lactam ring.
Dosing by species
- Susceptible infections · 22-40 mg/kg PO, SC, IM, or IV q8h · PO, SC, IM, IV · q8h
- Susceptible infections · 20-30 mg/kg IV q6-8h · IV · q6-8h · Dose extrapolated from adult horse data; use lower dose or longer interval in premature foals or those less than 7 days old.
- Susceptible infections · 25-50 mg/kg IM, IV twice daily · IM, IV · q12h
- Susceptible infections · 22-40 mg/kg PO, SC, IM, or IV q8h · PO, SC, IM, IV · q8h
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Known hypersensitivity to penicillins
- Oral administration in patients with septicemia, shock, or other grave illnesses (due to delayed/diminished GI absorption)
Adverse effects
- Anorexia
- Vomiting
- Diarrhea
- Antibiotic-associated diarrhea and superinfections
- Hypersensitivity reactions (rash, fever, eosinophilia, anaphylaxis)
- Neurotoxicity (ataxia, CNS effects at very high doses)
- Elevated liver enzymes
- Tachypnea
- Dyspnea
- Edema
- Tachycardia
Drug interactions
- Aminoglycosides · In vitro evidence of synergism against S. aureus strains; however, mixing in the same syringe/fluid line can cause physical inactivation.
- Cyclosporine · Oxacillin may reduce cyclosporine serum levels.
- Probenecid · Competitively blocks the tubular secretion of oxacillin, thereby increasing serum levels and prolonging serum half-life.
- Tetracyclines · Theoretical antagonism (bacteriostatic drugs may interfere with bactericidal action of penicillins); concurrent use is usually not recommended.
- Warfarin · Oxacillin may cause decreased warfarin efficacy.
Monitoring
- Clinical efficacy (resolution of infection)
- Signs of adverse effects (GI upset, allergic reactions)
Overdose
Acute oral overdoses are unlikely to cause significant problems other than **gastrointestinal distress** (vomiting, diarrhea). In humans, very high dosages of parenteral penicillins, especially in patients with compromised renal function, have induced **CNS effects and neurotoxicity**.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.