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).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways