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Antibiotic — IV Penicillin Pregnancy: Safe in pregnancy — well-established safety record; first-line IV penicillin in pregnancy

Benzylpenicillin (Penicillin G)

Brand names: Crystapen

Adult dose

Dose: 1.2–2.4 g
Route: IV
Frequency: Every 6 hours
Max: 24 g/day (meningitis doses); ENT: 1.2–2.4 g every 6 hours
IV antibiotic for severe ENT infections: peritonsillar abscess requiring admission, severe tonsillitis with sepsis, deep neck space infections, mastoiditis (combined with metronidazole). Switch to oral phenoxymethylpenicillin when clinically improved.

Paediatric dose

Dose: 25–50 mg/kg mg/kg
Route: IV
Frequency: Every 6 hours
Max: 2.4 g per dose
BNFc: neonates 25 mg/kg every 12h initially; children 25–50 mg/kg every 4–6h depending on severity

Dose adjustments

Renal

Reduce dose frequency in severe renal impairment (eGFR <10) — accumulation of penicillin G can cause seizures

Hepatic

No dose adjustment required

Paediatric weight-based calculator

BNFc: neonates 25 mg/kg every 12h initially; children 25–50 mg/kg every 4–6h depending on severity

Clinical pearls

  • IV first-line for sepsis-associated tonsillitis, mastoiditis, and deep neck space infections when oral therapy inadequate
  • High sodium content in some formulations — relevant in cardiac or renal patients on fluid restriction
  • Neurotoxicity and seizures at very high doses or in renal impairment — use caution and monitor
  • For mastoiditis: IV benzylpenicillin + metronidazole (anaerobic cover); early ENT review for potential mastoidectomy
  • Intrathecal administration is FATAL — never administer via this route; store IV preparations separately from intrathecal preparations

Contraindications

  • Penicillin hypersensitivity
  • Intrathecal administration (causes fatal neurotoxicity)

Side effects

  • Anaphylaxis
  • Electrolyte disturbance (high-dose — high sodium content)
  • Neurotoxicity/seizures (high dose or renal impairment)
  • GI disturbance
  • Rash

Interactions

  • Methotrexate — increased toxicity
  • Probenecid — increased penicillin levels

Monitoring

  • Renal function
  • Electrolytes (high dose)
  • Neurological status (high dose)

Reference: BNFc; BNF 90; BNFc; NICE NG84; ENT-UK Mastoiditis 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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