Long-acting penicillin 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: Tardocillin (specialist import)

Benzathine benzylpenicillin is a long-acting intramuscular depot formulation of penicillin used for syphilis, for primary and secondary prophylaxis of rheumatic fever, and for streptococcal infections requiring sustained cover.

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)

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 a poorly soluble salt that is slowly hydrolysed to release benzylpenicillin, which inhibits bacterial cell-wall synthesis by binding penicillin-binding proteins, giving prolonged low-level plasma concentrations from a single injection.

Prescribing in practice

Must be given by deep intramuscular injection only and never intravenously or near an artery or nerve, as inadvertent intravascular injection can cause serious cardiorespiratory reactions.
Contraindicated in penicillin hypersensitivity; have anaphylaxis management available as injections are given in a clinical setting.
In early syphilis warn of and prepare for a Jarisch-Herxheimer reaction shortly after the first dose.

Monitoring

Monitor for immediate injection-site and hypersensitivity reactions, and in syphilis follow up with serological titres to confirm treatment response.

Counselling the patient

This is a long-acting penicillin given as a deep injection into the muscle, often repeated at intervals for prophylaxis.
Tell the team about any previous penicillin allergy before the injection.
Fever, chills and a flu-like reaction may occur within hours of treatment for syphilis and usually settle.

Evidence & guidelines

It remains the recommended first-line treatment for most stages of syphilis and for secondary rheumatic fever prophylaxis in UK and international guidance.

Reference: BASHH UK syphilis guideline; UKHSA; SmPC; 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