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Long-Acting Penicillin — Syphilis / Rheumatic Fever Prophylaxis Pregnancy: Crosses the placenta; can be used during pregnancy when appropriately indicated with due consideration of benefits and risks. Excreted in breast milk in small amounts; stop breast-feeding if diarrhoea, candidosis or rash occurs in the child.

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.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: General therapy (adults and adolescents): 1.2 Million I.U. as a single dose; primary/secondary syphilis: 2.4 Million I.U. single dose
Route: Deep intramuscular injection only (upper outer quadrant of gluteus maximus or ventrogluteal field)
Frequency: Single dose (varies by indication)
Infectious diseases context. Dosing depends on severity/type of infection, age and hepato-renal function. Streptococcal disease: single injection of 0.6, 1.2 or 2.4 Million I.U. (10-day minimum course principle). Late-stage/latent syphilis: 2.4 Million I.U. once weekly for 3 weeks. Yaws and pinta: 1.2 Million I.U. single dose. Prophylaxis of rheumatic fever/poststreptococcal glomerulonephritis/erysipelas: 1.2 Million I.U. every 3-4 weeks, duration by cardiac involvement (at least 5-10 years or up to 21-40 years of age; life-long sometimes necessary). Children >=30 kg: 1.2 Million I.U.; children <30 kg: 0.6 Million I.U. Paediatric syphilis: 50,000 IU/kg body weight (max 2.4 Million I.U.). Congenital syphilis (neonates/infants, without neurological involvement): 50,000 IU/kg single dose. Max 5 ml ready-to-inject suspension per injection site. Note: source table values appear ambiguous in the extracted text; clinician to verify exact per-indication I.U. against the SPC.

Dose adjustments

Renal

Impaired renal function: 100% of normal daily dose if CrCl >=60 ml/min; 75% if CrCl 15-59 ml/min; 20-50% (max 1-3 Million I.U./day) if CrCl <15 ml/min. Can be removed by haemodialysis.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

  • Hypersensitivity to penicillins or any excipient
  • History of a severe immediate hypersensitivity reaction (e.g. anaphylaxis) to another beta-lactam agent (cephalosporin, carbapenem or monobactam)
  • When lidocaine solution is used as solvent, contraindications to lidocaine must be excluded

Side effects

  • Candidiasis (common)
  • Diarrhoea, nausea (common)
  • Allergic reactions, urticaria, angioedema; anaphylactic shock (rare)
  • Haemolytic anaemia, leukopenia, thrombocytopenia
  • Jarisch-Herxheimer reaction when treating syphilis; pain at injection site

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. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.