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# Benzathine Penicillin
## Overview
Benzathine penicillin G is a long-acting, parenteral depot formulation of penicillin G. It is slowly hydrolyzed to penicillin G, providing low but sustained serum concentrations over several weeks. It is intended for **intramuscular (IM) injection only**; it must never be administered intravenously (IV).
## Primary Indications
* Syphilis (primary, secondary, and tertiary/latent).
* Group A Streptococcal pharyngitis (prophylaxis or treatment when oral compliance is an issue).
* Rheumatic fever secondary prophylaxis.
## Adult Dosing
* **Syphilis (Primary, Secondary, or Early Latent):** 2.4 million units IM as a single dose.
* **Syphilis (Late Latent or Tertiary):** 2.4 million units IM weekly for 3 doses (total 7.2 million units).
* **Group A Strep Pharyngitis:** 1.2 million units IM as a single dose.
* **Rheumatic Fever Prophylaxis:** 1.2 million units IM every 3 to 4 weeks.
## Pediatric Dosing
* **Group A Strep Pharyngitis:**
* Patients <27 kg: 600,000 units IM as a single dose.
* Patients ≥27 kg: 1.2 million units IM as a single dose.
* **Syphilis (Congenital/Acquired):** Requires specialized weight-based dosing per current CDC guidelines (e.g., 50,000 units/kg IM as a single dose for primary/secondary; consult pediatric infectious disease protocols).
## Dose Adjustments
* **Renal Impairment:** Penicillin G is renally eliminated; however, due to the slow-release depot nature of the benzathine salt, adjustments are generally not required for standard, infrequent dosing. Use with caution in severe impairment due to potential accumulation of the active moiety.
## Contraindications
* History of severe hypersensitivity (anaphylaxis) to any penicillin.
* **NEVER administer IV, SC, or intrathecally.** Inadvertent IV administration can result in cardiopulmonary arrest or death.
## Adverse Effects
* **Common:** Injection site pain, sterile abscess, or induration.
* **Serious:** Anaphylaxis, Jarisch-Herxheimer reaction (fever, chills, headache during treatment of syphilis), and neurotoxicity (if accidentally injected into a vessel/nerve, specifically the sciatic nerve).
## Key Drug Interactions
* **Probenecid:** Decreases renal tubular secretion of penicillin, increasing serum concentrations (rarely relevant for depot benzathine, but clinically relevant for other penicillins).
* **Methotrexate:** Penicillins may decrease renal clearance of methotrexate, potentially increasing toxicity.
* **Live Vaccines:** High-dose penicillin may interfere with the immune response to certain live vaccines (e.g., BCG, typhoid).
## Monitoring
* **Post-Injection:** Monitor the patient for at least 30 minutes following administration for signs of hypersensitivity or anaphylaxis.
* **Syphilis:** Serologic monitoring (RPR/VDRL titers) at 6, 12, and 24 months to confirm treatment efficacy.
## Clinical Pearls
* **Injection Technique:** Administer via deep IM injection in the upper outer quadrant of the buttock (gluteal muscle). Use a long needle (e.g., 20–22 gauge) to ensure deep muscular delivery. Always aspirate before injecting to avoid intravascular administration.
* **Storage:** Keep refrigerated; do not freeze.
* **Local Protocols:** Dosing protocols for syphilis can vary based on regional resistance patterns and specific patient comorbidities (e.g., HIV status); always consult institutional ID guidelines.
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**Disclaimer:** This information is for educational purposes and does not replace professional clinical judgment. Verify all doses against current institutional protocols, prescribing information, and patient-specific factors before administration.
