Sexual Health / HIV/Penicillin (STI)

Benzathine penicillin (syphilis)

also: syphilis treatment · benzathine benzylpenicillin · Jarisch-Herxheimer

Overview

First-line for syphilis at every stage. The Jarisch–Herxheimer reaction and penicillin-allergy-in-pregnancy are the exam points.

Mechanism

Long-acting depot penicillin — inhibits cell-wall synthesis (PBP binding) → bactericidal against Treponema pallidum; depot maintains treponemicidal levels for days.

Indications

Early syphilis (single IM dose)
Late/latent syphilis (3 weekly doses)
Neurosyphilis (IV benzylpenicillin)
Congenital syphilis

Adverse effects

Jarisch–Herxheimer reactionclassic

Fever, myalgia, rash hours after the FIRST dose (released treponemal antigens) — counsel; NOT an allergy; antipyretics.

Injection-site paincommon

Penicillin hypersensitivity / anaphylaxisserious

Cautions & contraindications

Penicillin allergyall

Non-pregnant → doxycycline. PREGNANT + allergic → penicillin DESENSITISATION (doxycycline teratogenic; penicillin is the only reliable treponemicidal option crossing the placenta).

Interactions

Monitoring & kinetics

Non-treponemal titres (RPR/VDRL) to confirm response; partner notification; HIV test

IM depot (early); IV benzylpenicillin for neurosyphilis (depot doesn't reach CSF).

Choosing it

true

Source: BASHH 2015 — Syphilis