Narrow-spectrum penicillins: benzylpenicillin, benzathine benzylpenicillin and phenoxymethylpenicillin

Narrow-spectrum penicillins are mainly active against gram-positive bacteria, but are inactivated by beta-lactamase enzymes. Most Staphylococcus aureus strains produce beta-lactamase enzymes, and isolates should only be considered penicillin-susceptible (ie susceptible to benzylpenicillin and phenoxymethylpenicillin) if susceptibility is confirmed by a clinical microbiologist.

Phenoxymethylpenicillin (penicillin V) is intrinsically less active than benzylpenicillin.

Benzylpenicillin (penicillin G) remains the treatment of choice for susceptible infections requiring intravenous therapy (such as pneumococcal pneumonia) because of its narrow spectrum of activity.

Benzathine benzylpenicillin is given intramuscularly. A dose of benzathine benzylpenicillin provides low blood concentrations of benzylpenicillin for up to 4 weeks. The ventrogluteal site is preferred for administration of intramuscular benzathine benzylpenicillin because of reduced pain and risk of nerve injury; for administration advice, see Instructions for intramuscular injection at the ventrogluteal site. The vastus lateralis (anterolateral thigh) is also an acceptable site and may be preferred in neonates, infants and small children; for administration advice, see Safer Care Victoria guidance on Intramuscular injections for neonates.

Figure 1. Instructions for intramuscular injection at the ventrogluteal site. [NB1]
  1. Place the patient in a side-lying position.
  2. Place your right hand on the patient’s left hip, or left hand on the patient’s right hip.
  3. With the palm of your hand, locate the greater trochanter of the femur.
  4. Place your index finger towards the front or anterior superior iliac spine, and fan your middle finger as far along the iliac crest as you can reach. Your thumb should always be pointed towards the front of the patient’s leg.
  5. The injection site is in the middle of the triangle between your middle and index fingers.
  6. Remove your fingers before inserting the needle.

Note:

NB1: For a video on the intramuscular administration of benzathine benzylpenicillin at the ventrogluteal site, see the RHDAustralia website.

Source: RHDAustralia (ARF/RHD writing group). The 2020 Australian guideline for prevention, diagnosis and management of acute rheumatic fever and rheumatic heart disease (3rd edition). Darwin, NT: RHD Australia; 2020. [URL]