Cloxacillin
- **Class:** Narrow-spectrum, penicillinase-resistant penicillin (isoxazolyl-penicillin). - **Spectrum:** Highly effective against penicillinase-producing gram-positive cocci, particularly **Staphylococcus** species. - **Limitations:** Inactive against *Rickettsia*, mycobacteria, fungi, *Mycoplasma*, and viruses. Methicillin-resistant staphylococci (MRSA/MRSP) are inherently resistant. - **Availability:** In the USA, only intramammary infusion products are currently marketed for veterinary use (dairy cattle). Oral formulations are no longer commercially available in the US, though they may be compounded or available internationally.
Mechanism: Cloxacillin is a bactericidal beta-lactam antibiotic. Binds to **Penicillin-Binding Proteins (PBPs)** โ inhibits peptidoglycan cross-linking โ disrupts bacterial cell wall synthesis โ cell lysis and death. **Mechanistic Pearl:** The bulky isoxazolyl side chain sterically hinders staphylococcal **beta-lactamases**, preventing the enzymatic hydrolysis of the beta-lactam ring that normally inactivates natural penicillins.
Dosing by species
- Susceptible infections ยท 20-40 mg/kg PO or IM q6-8h ยท PO/IM ยท q6-8h
- Staph. pyoderma, diskospondylitis, osteoarthritis or skin infections ยท 10-15 mg/kg PO q6h for 14-84 days ยท PO ยท q6h ยท 14-84 days
- Systemic infections, bacteremia ยท 10-40 mg/kg PO q6-8h for 7-14 days ยท PO ยท q6-8h ยท 7-14 days
- Ocular infections ยท Apply 1/10 of a tube (0.3 g) ยท topical ยท q24h ยท Specifically for beta-lactamase-producing Staphylococcus
- Mastitis (treatment or prophylaxis) caused by susceptible organisms - Lactating cow ยท After milking out and disinfecting teat, instill contents of syringe; massage. ยท Intramammary ยท q12h ยท 3 total doses ยท Using lactating cow formula (Dari-Clox)
- Mastitis (treatment or prophylaxis) caused by susceptible organisms - Dry (non-lactating) cows ยท After last milking (or early in the dry period), instill contents of syringe and massage into each quarter. ยท Intramammary ยท Once ยท Using dry cow formula (benzathine)
- Susceptible infections ยท 20-40 mg/kg PO q8h ยท PO ยท q8h ยท Oral dosage forms are no longer available commercially in the USA
- Staph. pyoderma, diskospondylitis, osteoarthritis or skin infections ยท 10-15 mg/kg PO q6h for 14-84 days ยท PO ยท q6h ยท 14-84 days
Routes of administration
Contraindications
- Known hypersensitivity to penicillins
- Known hypersensitivity to penicillins or cephalosporins
Adverse effects
- Hypersensitivity reactions (rashes, fever, anaphylaxis)
- Eosinophilia
- Neutropenia
- Agranulocytosis
- Thrombocytopenia
- Leukopenia
- Anemias
- Lymphadenopathy
- Hypersensitivity reactions
- Local irritation (ophthalmic)
Monitoring
- Clinical efficacy (resolution of infection/mastitis)
- Signs of hypersensitivity or toxicity
- Resolution of clinical signs of ocular infection
- Signs of local irritation or hypersensitivity
Overdose
Overdosage of intramammary infusions is unlikely to pose much systemic risk to the patient. > **Note:** Excessive dosing may prolong required withdrawal times for milk and meat in food-producing animals.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.