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Beta-lactam antibiotic (natural penicillin) Pregnancy: Safe in pregnancy.

Benzylpenicillin (Penicillin G)

Brand names: Crystapen

Adult dose

Dose: Meningitis/Meningococcaemia: 1.2 g IV every 4 hours; Severe infections: 0.6–2.4 g every 4–6 hours
Route: IV (infusion over 15–30 min) or IM
Frequency: Every 4–6 hours
Max: 14.4 g/day
Suspected meningococcal disease (pre-hospital or A&E): 1.2 g IV or IM immediately. Meningitis: 2.4 g IV every 4 hours. Streptococcal sepsis: 1.2–2.4 g every 4–6 hours. Infective endocarditis: 1.2 g every 4 hours.

Paediatric dose

Dose: 50 mg/kg
Route: IV or IM
Frequency: Every 4–6 hours
Max: 2.4 g/dose
Concentration: 60 mg/ml
Meningococcal disease: neonates 50 mg/kg every 12h (up to 100 mg/kg/day); 1 month–17 years: 50 mg/kg every 4–6 hours (max 2.4 g/dose). Pre-hospital GP dose: 300 mg (<1 year), 600 mg (1–9 years), 1200 mg (≥10 years) IM.

Dose adjustments

Renal

Reduce frequency if eGFR <10 (every 8–12 hours); accumulation causes seizures.

Hepatic

No dose adjustment required.

Paediatric weight-based calculator

Meningococcal disease: neonates 50 mg/kg every 12h (up to 100 mg/kg/day); 1 month–17 years: 50 mg/kg every 4–6 hours (max 2.4 g/dose). Pre-hospital GP dose: 300 mg (<1 year), 600 mg (1–9 years), 1200 mg (≥10 years) IM.

Clinical pearls

  • Give immediately if meningococcal disease suspected — do not wait for LP
  • NICE NG240: do not delay antibiotics for imaging
  • Pre-hospital IM dose if IV access not possible — 1.2 g IM in adults
  • Ceftriaxone used in hospitals; benzylpenicillin in pre-hospital by GPs under NICE guidance
  • Highest sodium load with sodium benzylpenicillin (1.68 mmol/600 mg) — monitor Na+ in high doses

Contraindications

  • Penicillin hypersensitivity (anaphylaxis)

Side effects

  • Anaphylaxis (0.01–0.05%)
  • Rash
  • Seizures (high doses, renal failure)
  • Electrolyte disturbance (high sodium/potassium load with large IV doses)
  • Jarisch-Herxheimer reaction (spirochaete infections)

Interactions

  • Methotrexate — reduced excretion
  • Probenecid — increases penicillin levels

Monitoring

  • Clinical response
  • Seizure activity (high doses/renal failure)
  • Electrolytes (Na+/K+)

Reference: BNFc; BNF; NICE NG240; PHE Meningococcal Guidelines. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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