Disclaimer to the On-line Edition
This Manual has been designed for use in the NICU at London Health Sciences Centre (LHSC), London, Ontario, Canada, and represents clinical practice at this institution. The information contained within the Manual may not be applicable to other centres. If users of this Manual are not familiar with a drug, it is recommended that the official monograph be consulted before it is prescribed and administered. Any user of this information is advised that the contributors, Editor and LHSC are not responsible for any errors or omissions, and / or any consequences arising from the use of the information in this Manual.



  • mainly used for the treatment of infections caused by, or suspected of being caused by susceptible, penicillinase - producing staphylococci (eg. cutaneous infections, septicemia, endocarditis and osteomyelitis)


  • cloxacillin is a semisynthetic, penicillinase - resistant penicillin
  • penicillinase - resistant penicillins have their greatest in vitro antimicrobial activity against Gram-positive aerobic cocci (S. aureus, S. saprophyticus; groups A, B, C and G streptococci).
  • Enterococci, including S. faecalis, are generally resistant
  • cloxacillin works like other penicillins: ie. inhibition of bacterial cell wall synthesis
  • in healthy, fasting adult volunteers, only 37-60% of an orally administered dose was absorbed; the maximum serum concentration following oral administration in neonates would be expected to occur at about 1 hour
  • cloxacillin distributes well into body fluids (synovial fluid, pleural fluid, bile) and tissues (liver, kidneys); however, it penetrates poorly into the CSF
  • in adults, approximately 20% of the drug is metabolized in the liver; the remainder of the dose is eliminated as unchanged drug and active metabolites by the kidneys

Side Effects

  • hypersensitivity reactions (rash, eosinophilia, hemolytic anemia); these reactions are rare in newborns
  • hematological effects (including transient neutropenia, thrombocytopenia)
  • gastrointestinal effects (vomiting, diarrhea, gastritis) following oral administration


  • may be given IV or orally

    < 2,000 g< 7 days
    > 7 days
    25 mg/kg q12h
    25 mg/kg/q8h
    >2,000 g< 7 days
    > 7 days
    25 mg/kg q8h
    25 mg/kg q6h


  • 50 mg/mL syringe prepared by Pharmacy
  • 25 mg/mL oral suspension (after reconstitution); keep refrigerated
  1. Add 4.8 mL Sterile Water to the 500 mg vial. This provides a concentration of 100 mg/mL.
  2. Take 5 mL of the 100 mg/mL strength (i.e., 500 mg) and add to 5 mL of diluent. This provides a final concentration of 500 mg/10 mL (= 50 mg/mL) which can then be administered to the baby.


  1. McEvoy G K (ed): AHFS Drug Information, American Society of Hospital Pharmacists, 1991.
  2. Perlman M and Kirpalani H (eds): Residents Handbook of Neonatology (The Hospital for Sick Children, Toronto, Canada), Mosley Year Book, Toronto, 1992.
  3. Philip AGS: Neonatology - A Practical Guide, W B Saunders Company, Toronto, 1987.

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