Procaine benzylpenicillin !
A procaine salt of penicillin G for intramuscular injection, shorter-acting than the benzathine salt; it can cause a brief neuropsychiatric reaction.
Updated 2026-11-01
Also called: penicillin G procaine, procaine penicillin
The figure shows the DD-peptidase of Streptomyces R61, the classic PBP model, acylated by penicillin G (the active molecule the procaine salt releases).
Mechanism of action
The procaine salt of penicillin G forms an intramuscular depot that releases penicillin G over hours, longer than soluble penicillin G but much shorter than the benzathine salt. Penicillin G acylates the PBPs of streptococci, Treponema and other susceptible organisms. The released procaine can cause an immediate brief reaction of anxiety, confusion or hallucinations.
- The intramuscular injection forms a depot
- The depot releases penicillin G over hours
- Penicillin G acylates the PBPs
- Dividing bacteria lyse
Pharmacokinetics
- Absorption
- Slow from muscle; useful levels for hours after each injection.
- Elimination
- The released penicillin is cleared by the kidney.
Indications
- Mild to moderate infections by susceptible streptococci, such as pharyngitis. Streptococcal pharyngitis →
- Some forms of syphilis, depending on the stage and guidelines. Syphilis →
Adverse effects
Common
- Pain, lump or bruising at the injection site
- Immediate procaine reaction: anxiety, agitation, confusion, hallucinations or seizures, lasting minutes
Contraindications
- Allergy to penicillin or to procaine
- It is not a treatment for gonorrhoea
Key interactions
- Probenecid
- Reduces tubular secretion: higher and longer-lasting β-lactam levels.
- Tetracyclines and chloramphenicol
- Bacteriostatic drugs can antagonise the bactericidal effect.
What is monitored
- Observe the patient after each injection
- In syphilis, serological response
Administration and safety
- Deep intramuscular only, in the buttock or thigh; never intravenous.
- Do not confuse with benzathine benzylpenicillin or the mixture of both: duration and uses differ.
- Aspirate before injecting.
Nursing care
- Before the first dose: ask about allergy to penicillins, cephalosporins and other β-lactams, and what the reaction was.
- Also ask about allergy to procaine or other local anaesthetics.
- Keep the patient under observation: if the procaine reaction occurs, reassure, protect from injury and report.
- After the first dose, watch for hives, wheeze, low blood pressure or swelling of lips and tongue, with anaphylaxis equipment at hand.
Pregnancy and breastfeeding
Penicillins are widely used in pregnancy without a known rise in birth defects.
Test yourself
Think of the answer, then tap the question to see it.
What is the immediate reaction that can follow a procaine penicillin G injection?
A brief neuropsychiatric reaction to procaine (anxiety, confusion, hallucinations) that lasts minutes and is not a penicillin allergy.
How does it differ from benzathine benzylpenicillin?
It releases penicillin over hours, not weeks, and reaches higher levels; the benzathine salt is the longest-acting.
Is procaine penicillin used for gonorrhoea today?
No: many gonococcal strains are resistant to penicillin.
Related drugs
Related conditions
← Back to the class: Beta-lactams
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: Penicillin G Procaine Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); Syphilis - STI Treatment Guidelines (Centers for Disease Control and Prevention (CDC), 2021); Farmacología básica y clínica (Katzung), cap. 43 (McGraw Hill, 2025).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.