Oxacillin
Oxacillin sodium
Also known as: Bactocill · Sodium oxacillin · Methylphenyl isoxazolyl penicillin · Oxacillinum natricum · P-12 · SQ-16423
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Susceptible infections | 22-40 mg/kg PO, SC, IM, or IV q8h | PO, SC, IM, IV | q8h | — | 🌎 NA |
Cats
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Susceptible infections | 22-40 mg/kg PO, SC, IM, or IV q8h | PO, SC, IM, IV | q8h | — | 🌎 NA |
Horses
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Susceptible infections | 20-30 mg/kg IV q6-8h | IV | q6-8h | — | 🌎 NA |
| Susceptible infections | 25-50 mg/kg IM, IV twice daily | IM, IV | q12h | — | 🌎 NA |
- Susceptible infections: Dose extrapolated from adult horse data; use lower dose or longer interval in premature foals or those less than 7 days old.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
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 of action
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.
Safety & warnings
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
- 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
Precautions
Use cautiously in patients with documented hypersensitivity to other beta-lactam antibiotics (e.g., cephalosporins, carbapenems) due to potential cross-reactivity. Safe use during pregnancy is not firmly established, though it is generally considered safe (Papich Class A / FDA Category B). May cause diarrhea, candidiasis, or allergic responses in nursing offspring as it is excreted in maternal milk.
Drug interactions
In vitro evidence of synergism against S. aureus strains; however, mixing in the same syringe/fluid line can cause physical inactivation.
Oxacillin may reduce cyclosporine serum levels.
Competitively blocks the tubular secretion of oxacillin, thereby increasing serum levels and prolonging serum half-life.
Theoretical antagonism (bacteriostatic drugs may interfere with bactericidal action of penicillins); concurrent use is usually not recommended.
Oxacillin may cause decreased warfarin efficacy.
Monitoring
- Clinical efficacy (resolution of infection)
- Signs of adverse effects (GI upset, allergic reactions)
Pharmacokinetics
Half-life
Absorption
Resistant to acid inactivation in the gut but only partially absorbed orally (30-35% bioavailability in humans). Food decreases both the rate and extent of absorption. Rapidly absorbed after IM administration (peak levels within 30 minutes).
Distribution
Distributes to lungs, kidneys, bone, bile, pleural fluid, synovial fluid, and ascitic fluid. Volume of distribution is 0.3 L/kg in dogs. Minimal CSF penetration unless meninges are inflamed. Highly protein-bound (89-94% in humans).
Metabolism
Partially metabolized in the liver to both active and inactive metabolites.
Elimination
Rapidly excreted in the urine via both glomerular filtration and tubular secretion. A small amount is excreted in feces via biliary elimination.
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.
Available products
Formulations
- Powder for oral solution
- Powder for injection
Human-labeled
- Oxacillin Sodium Powder for Oral Solution: 250 mg/5 mL (when reconstituted)
- Oxacillin Sodium Powder for Injection: 500 mg, 1 gram, 2 grams, 10 grams
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Store unmixed powder at room temperature (15-30°C) in tight containers. Reconstituted oral solution: refrigerate and discard after 14 days (stable for 3 days at room temperature). Reconstituted injection (167 mg/mL): stable for 3 days at room temperature or 7 days if refrigerated.
Client information
Important Dosing Schedule: This medication is eliminated from the body very quickly and must be given exactly as prescribed (usually every 8 hours) to remain effective.
- Administration: Give this medication on an empty stomach, at least 1 hour before feeding or 2 hours after a meal, unless your veterinarian instructs otherwise.
- Storage: Keep the oral liquid suspension in the refrigerator. Discard any unused medication after 14 days.
- Side Effects: Watch for vomiting, diarrhea, or loss of appetite. Contact your veterinarian if these occur or if you notice any signs of an allergic reaction (facial swelling, hives, difficulty breathing).
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.