Benzylpenicillin (Penicillin G)

How it works

It inhibits bacterial cell-wall synthesis by binding penicillin-binding proteins, and is bactericidal against susceptible Gram-positive cocci such as group A streptococci.

Prescribing in practice

• Contraindicated in patients with previous immediate hypersensitivity to penicillins; check allergy status carefully and have anaphylaxis management available, as serious reactions can occur with parenteral dosing.
• It is acid-labile and not absorbed orally, so it must be given by injection; doses are reduced in significant renal impairment because it is renally cleared.
• Very high parenteral doses contribute a notable sodium and potassium load and have rarely been associated with neurotoxicity (seizures), particularly in renal impairment.

Monitoring

Monitor renal function, electrolytes and clinical response, and observe for hypersensitivity, especially after the first parenteral dose.

Counselling the patient

• Tell staff at once about any rash, swelling, wheeze or faintness during or after the injection.
• Report any previous reaction to penicillin or other antibiotics before treatment.
• Complete the full prescribed course as directed.

Evidence & guidelines

Penicillin G remains a first-line agent for invasive group A streptococcal and meningococcal disease in national antimicrobial and infection guidance.