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Long-Acting Penicillin — Syphilis / Rheumatic Fever Prophylaxis

Benzathine Benzylpenicillin

Brand names: Bicillin L-A, Penidure LA

Used in: Meningitis & Encephalitis

Benzathine benzylpenicillin is a long-acting intramuscular beta-lactam antibiotic used principally for syphilis and for the treatment and secondary prevention of streptococcal disease and rheumatic fever.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

Clinical monograph

How it works

It is bactericidal, binding penicillin-binding proteins to inhibit bacterial cell-wall peptidoglycan cross-linking, leading to cell lysis.

Prescribing in practice

  • It must never be given intravenously or by any route other than deep intramuscular injection, as inadvertent intravascular administration can be fatal.
  • Penicillin hypersensitivity, including a history of anaphylaxis, is an absolute contraindication, so confirm allergy status before administration.
  • A Jarisch-Herxheimer reaction may follow the first dose when treating syphilis and should be anticipated and explained.

Monitoring

No routine laboratory monitoring is required; observe the patient after injection for immediate hypersensitivity reactions.

Counselling the patient

  • This is a deep injection into the muscle and a degree of injection-site soreness is common.
  • Tell the clinician immediately about any rash, swelling, wheeze or feeling faint.
  • For syphilis, a short flu-like reaction can occur within the first day of treatment and usually settles.

Evidence & guidelines

Benzathine benzylpenicillin remains the established first-line treatment for syphilis in UK and international guidelines.

Reference: BASHH Syphilis Guidelines 2019; NICE STI Guidelines; WHO Syphilis Treatment Guidelines 2016; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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