Penicillin G (Benzylpenicillin) is a narrow-spectrum, bactericidal, beta-lactam antibiotic derived from Penicillium fungi. It is the first natural penicillin and remains a first-line agent for susceptible Gram-positive organisms, particularly Streptococcus pyogenes, Streptococcus pneumoniae, and sensitive strains of Staphylococcus aureus. It is also the drug of choice for syphilis, actinomycosis, and clostridial infections. In the Indian context, it is a critical, cost-effective antibiotic for severe community-acquired infections, though resistance patterns necessitate sensitivity testing.
Adult: Varies by infection. Moderate: 1-4 MU IV every 4-6 hours. Severe (e.g., endocarditis, meningitis): 18-24 MU IV daily in divided doses every 4 hours. Neurosyphilis: 18-24 MU IV daily as 3-4 MU every 4 hours for 10-14 days. Uncomplicated syphilis: 2.4 MU IM as single dose (as Benzathine Penicillin).
Note: For IV use: Reconstitute 1 MU vial with at least 10 mL of Sterile Water for Injection or Normal Saline. Further dilute in 50-100 mL of compatible IV fluid (NS, D5W) and infuse over 30-60 minutes to avoid CNS irritation. For IM use: Reconstitute with Sterile Water for Injection or 1% Lidocaine HCl (without epinephrine) to reduce pain. Administer deep IM into a large muscle mass. NEVER give IV push.
Penicillin G binds to specific penicillin-binding proteins (PBPs) located on the inner cytoplasmic membrane of the bacterial cell wall. This binding inhibits the final transpeptidation step of peptidoglycan synthesis in the bacterial cell wall, a critical structural component. The inhibition leads to the activation of autolytic enzymes (autolysins) in the cell wall, resulting in bacterial cell lysis and death.
Pregnancy: US FDA Category B. Considered safe and is the drug of choice for syphilis in pregnancy. Crosses the placenta. Use for clear indications.
Driving: No direct effect. However, if CNS side effects like dizziness or seizures occur, patient should not drive or operate machinery.
| Probenecid | Competitively inhibits renal tubular secretion of penicillin, increasing and prolonging serum levels. | Moderate |
| Methotrexate | Penicillins may reduce renal clearance of methotrexate, increasing risk of toxicity. | Major |
| Warfarin | May potentiate anticoagulant effect by altering gut flora and reducing vitamin K production; monitor INR closely. | Moderate |
| Oral Contraceptives | Theoretical reduced efficacy due to altered enterohepatic circulation; advise backup contraception. | Minor |
| Aminoglycosides (e.g., Gentamicin) | In vitro synergistic against some organisms (e.g., Enterococcus), but physical incompatibility - do not mix in same IV line. | Moderate |
| Tetracyclines, Chloramphenicol | Bacteriostatic agents; may antagonize the bactericidal effect of penicillin. | Moderate |
Same composition (Penicillin G (1000000IU)), different brands: