1. Clinical Overview
Benzathine penicillin G is a long-acting, repository form of penicillin G, a natural penicillin antibiotic. It is a combination of one mole of penicillin G and two moles of benzathine, resulting in a very low solubility. This allows for a sustained release of penicillin G from the intramuscular injection site, providing detectable serum levels for up to 4 weeks. It is a cornerstone in public health programs in India for the prevention and treatment of specific bacterial infections, particularly group A streptococcal pharyngitis (to prevent rheumatic fever) and syphilis.
| Onset | Duration | Bioavailability |
|---|---|---|
| Slow; detectable serum levels within 4-6 hours of IM injection. | Prolonged; therapeutic levels against susceptible streptococci can persist for 1-4 weeks. Treponemicidal levels (>0.03 IU/mL) are maintained for approximately 2-3 weeks. | Approximately 100% from intramuscular injection, but absorption is very slow and rate-limited. |
2. Mechanism of Action
Benzathine penicillin G, after hydrolysis to penicillin G, acts as a bactericidal agent against susceptible microorganisms during active multiplication. It inhibits the third and final stage of bacterial cell wall synthesis by binding to specific penicillin-binding proteins (PBPs) located inside the bacterial cell wall. This binding inhibits the transpeptidase enzyme, preventing cross-linking of peptidoglycan chains, which is essential for cell wall strength and rigidity. The osmotic imbalance leads to cell lysis and death.
3. Indications & Uses
- Prophylaxis of rheumatic fever recurrence (secondary prevention)
- Treatment of early syphilis (primary, secondary, and early latent syphilis of <2 years duration)
- Treatment of group A streptococcal (Streptococcus pyogenes) pharyngitis/tonsillitis (when oral therapy is not feasible or compliance is a concern)
4. Dosage & Administration
Adult Dosage: Rheumatic Fever Prophylaxis: 1,200,000 IU IM every 3-4 weeks or 2,400,000 IU IM every 4 weeks. Early Syphilis: 2,400,000 IU IM as a single dose. Strep Pharyngitis: 1,200,000 IU IM as a single dose (or 2,400,000 IU for larger adults).
Administration: FOR DEEP INTRAMUSCULAR INJECTION ONLY. Use the upper outer quadrant of the gluteal region. In adults, the ventrogluteal site is preferred. Aspirate before injection to avoid intravascular administration. Administer slowly. The suspension is thick; use a 19- or 20-gauge needle. Shake the multi-dose vial vigorously before withdrawing the dose. Do not mix with other medications.
5. Side Effects
Common side effects may include:
- Pain, tenderness, and induration at the injection site
- Mild local inflammatory reaction
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Probenecid | Markedly increases and prolongs serum penicillin levels by blocking renal tubular secretion. | Moderate |
| Methotrexate | Penicillins may reduce renal clearance of methotrexate, increasing risk of toxicity. | Major |
| Oral Contraceptives | Potential decreased efficacy of oral contraceptives; advise use of backup non-hormonal contraception. | Moderate |
| Warfarin | Possible increased anticoagulant effect; monitor INR closely. | Moderate |
| Aminoglycosides (e.g., Gentamicin) | In vitro antagonism may occur if mixed together; administer separately. | Minor |
7. Patient Counselling
- Do inform your doctor about any past allergic reactions to penicillin or other drugs.
- Do complete the full course of prophylaxis as prescribed, even if you feel well.
- Do expect pain and a lump at the injection site for a few days; this is normal.
- DO NOT allow this medicine to be given into a vein or artery.
- Do inform all healthcare providers (including dentists) that you are on penicillin prophylaxis.
8. Toxicology & Storage
Overdose: Primarily CNS toxicity due to accumulation, especially in renal impairment: confusion, lethargy, myoclonus, seizures, encephalopathy. Electrolyte imbalance (hypernatremia, hyperkalemia from the potassium salt in some formulations) is possible with massive overdose.
Storage: Store at 2°C to 8°C (in a refrigerator). Do not freeze. Protect from light. The suspension is stable for up to 12 months under refrigeration. Once reconstituted (if applicable), the multi-dose vial should be used within the time period specified by the manufacturer (usually 7-14 days if stored refrigerated). Keep out of reach of children.