Bicillin CR vs LA



Last week we discussed the value of the tried and trusted antibiotic, Penicillin, for pneumonia. In the past we have also mentioned Acute Rheumatic Fever and the management of Strep Pharyngitis. We know that the therapy of choice for Strep Pharyngitis is Penicillin, and many of us prefer to use intramuscular injection.  Often, though, the question comes up as to which form of the IM Penicillin is appropriate: Bicillin® CR or Bicillin® LA?


Historic Perspective

  • Penicillin G first became available for use in the 1940s.
  • Penicillin salts given intravenously or intramuscularly were known to cause a lot of pain and tissue damage.
    • They also had a short 1/2 life.
    • The short 1/2 life meant you had to administer it frequently! Not awesome for a painful medication.
  • Penicillin became the recommended therapy for Group A beta-hemolytic streptococci and strep pharyngitis.
    • This therapy was associated with reduced rates of rheumatic fever and associated rheumatic heart disease.
    • Now Kawasaki Disease is the leading cause of acquired heart disease in the developed countries, because of successful therapy to treat rheumatic fever.
    • Therapy is recommended for 10 DAYS!  That is a problem when you have a short 1/2 life medication!
  • Penicilins with longer 1/2 lives
    • Procaine Penicillin
      • Repository form of intramuscular penicillin G that became available in 1948.
      • PROS
        • Had local anesthetic effects.
        • Was well tolerated and had less adverse effects.
        • Had a prolonged therapeutic window – maintained appropriate blood levels for 24 hours.
      • CONS
        • Shorter 1/2 life makes it inadequate for treatment of strep pharyngitis as a single dose.
        • Would still require 10 IM shorts (one q day for 10 days).
    • Benzathine Penicillin
      • Repository form of intramuscular penicillin G that became available in 1952.
      • PROS
        • Produced prolonged blood levels for up to 4 WEEKS!
        • Had high cure rates and, when given monthly, found to prevent recurrence of rheumatic fever.
        • Became the Gold Standard for treating strep pharyngitis and rheumatic fever.
      • CONS
        • “Shot of Penicillin” became synonymous with PAIN!
        • IM shots of Benzathine Penicillin lead to significant pain and tenderness.
        • The pain was noted to be so severe in children, that many would refuse to walk for 2-3 days after injection.

Bicillin® CR

  • Combination of Benzathine PCN and Procaine PCN
    • Comes in mixture of 900 Units of Benzathine PCN and 300 Units of Procaine PCN or
    • Mixture of 1.2 Million Units of Benzathine PCN and 1.2 Million Units of Procaine PCN
  • Found to have significantly lower incidence of pain and tenderness, while maintaining effective therapeutic levels for the treatment of strep pharyngitis.


  • So why would you use LA instead of CR?
    • In larger children, you may need to split the dose into two separate injections… but more importantly,
    • Bicillin® CR is NOT appropriate therapy for Syphilis!
      • There have been reported cases of Syphilis being inadequately treated with Bicillin® CR.
      • Prolonged serum concentration is needed for treating syphilis effectively, because the spirochete slowly replicates.
      • For this Bicillin® LA is required.
        • Contains either 2.4 Million Units of Benzathine PCN.
        • Bicillin® CR packaging is now labeled with a warning “Not indicated for the treatment of syphilis.”

Moral of the Morsel:

  • Select Bicillin® CR when treating strep pharyngitis.
  • Select Bicillin® LA when treating syphilis… henceforth referred to as syphi-LA-s.




Bycroft TC1, O’Connor T, Hoff C, Bohannon A. When choosing injectable penicillin for the treatment of group A beta-hemolytic streptococcal pharyngitis, there is a less painful choice. Pediatr Emerg Care. 2000 Dec;16(6):398-400. PMID: 11138880. [PubMed] [Read by QxMD]

Bass JW. A review of the rationale and advantages of various mixtures of benzathine penicillin G. Pediatrics. 1996 Jun;97(6 Pt 2):960-3. PMID: 8637782. [PubMed] [Read by QxMD]


Sean M. Fox
Sean M. Fox
Articles: 583


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