PFIZERPEN G
Clinical safety rating: caution
Comprehensive clinical and safety monograph for PFIZERPEN G (PFIZERPEN G).
Inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), inhibiting transpeptidase activity and autolysin inhibition.
| Metabolism | Primarily renally eliminated; undergoes tubular secretion with minimal hepatic metabolism. |
| Excretion | Primarily renal excretion via glomerular filtration and tubular secretion; 60-90% of dose excreted unchanged in urine within 6 hours. Biliary excretion accounts for less than 10%. |
| Half-life | Terminal elimination half-life is 30-60 minutes in patients with normal renal function; prolonged to 2-10 hours in renal impairment (CrCl <10 mL/min). |
| Protein binding | Approximately 60-80% bound to serum albumin. |
| Volume of Distribution | 0.2-0.3 L/kg; limited distribution primarily to extracellular fluid; low penetration into CSF unless meninges inflamed (CSF concentrations 5-10% of serum). |
| Bioavailability | Intramuscular: 60-100% (variable) due to slow absorption. Oral: 15-30% (unreliable, therefore not used). |
| Onset of Action | Intravenous: immediate (peak plasma concentration within minutes). Intramuscular: 15-30 minutes. Subcutaneous: 30-60 minutes. |
| Duration of Action | Intravenous: 2-4 hours (bactericidal levels maintained). Intramuscular: 4-6 hours. Sustained release formulations (e.g., procaine penicillin G) may extend to 12-24 hours. |
| Molecular Weight | 372.48 |
2-4 million units (1.2-2.4 g) IV every 4-6 hours; maximum 24 million units per day.
| Dosage form | TABLET |
| Renal impairment | GFR 10-50 mL/min: 2-4 million units IV every 6-8 hours; GFR <10 mL/min: 2-4 million units IV every 8-12 hours. |
| Liver impairment | No adjustment required; primarily renally excreted. |
| Pediatric use | Neonates <7 days: 50,000-100,000 units/kg IV every 12 hours; infants and children: 100,000-250,000 units/kg/day IV divided every 4-6 hours. |
| Geriatric use | Dose based on renal function; consider lower initial doses due to age-related decline in GFR. |
| 1st trimester | Penicillin G is generally considered safe during the first trimester. Animal studies have not shown teratogenic effects, and there is no evidence of increased risk of congenital anomalies in humans. However, use only if clearly needed. |
| 2nd trimester | Penicillin G is generally considered safe during the second trimester. It is the drug of choice for treating syphilis during pregnancy. No known risks to the fetus when used at recommended doses. |
| 3rd trimester | Penicillin G is generally considered safe during the third trimester. It is used to prevent group B streptococcal transmission to the newborn during delivery. No known risks to the fetus or neonate. |
Clinical note
Comprehensive clinical and safety monograph for PFIZERPEN G (PFIZERPEN G).
| Placental transfer | Penicillin G crosses the placenta readily, achieving therapeutic concentrations in fetal serum and amniotic fluid. It is actively transported across the placenta. |
| Breastfeeding |
■ FDA Black Box Warning
No FDA black box warning.
| Serious Effects |
Hypersensitivity to penicillins or any component of the formulationHistory of severe immediate hypersensitivity reaction (e.g., anaphylaxis) to beta-lactam antibiotics
| Precautions | Severe hypersensitivity reactions (anaphylaxis, angioedema), Clostridioides difficile-associated diarrhea (CDAD), Neurotoxicity with high doses or renal impairment (seizures, myoclonus), Electrolyte disturbances (sodium or potassium overload with high doses), Renal impairment requires dose adjustment, Use in neonates requires caution due to immature renal function |
| Food/Dietary | No significant food interactions. However, oral absorption of oral penicillin V is decreased with food; administer on an empty stomach (1 hour before or 2 hours after meals). For injectable penicillin G, food does not affect administration. |
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| Penicillin G is excreted into breast milk in low concentrations, which are unlikely to cause adverse effects in the nursing infant. However, potential risks include alteration of gut flora, diarrhea, and allergic sensitization. Use with caution in mothers of infants with known hypersensitivity to penicillins. |
| Lactation Rating | L1 (Compatible) |
| Teratogenic Risk | Penicillin G (PFIZERPEN G) is classified as FDA Pregnancy Category B. Animal reproduction studies have not demonstrated a risk to the fetus, but there are no adequate and well-controlled studies in pregnant women. Generally considered low risk; use only if clearly needed. No known teratogenic effects in first trimester; second and third trimester use is considered safe. |
| Fetal Monitoring | No specific routine monitoring required beyond standard obstetric care. Monitor maternal renal function if high doses are used, as excretion is renal. Observe for signs of allergic reaction or anaphylaxis in the mother. Fetal monitoring per standard obstetric indications. |
| Fertility Effects | No known adverse effects on fertility in animal or human studies. Penicillin G is not associated with impairment of reproductive function. |
| Clinical Pearls | PFIZERPEN G (penicillin G) is a narrow-spectrum penicillin primarily active against gram-positive cocci (e.g., Streptococcus pyogenes, Streptococcus pneumoniae) and some gram-negative cocci (e.g., Neisseria meningitidis). It is the drug of choice for syphilis (Treponema pallidum). For treatment of group A streptococcal pharyngitis, a single dose of benzathine penicillin G 1.2 million units IM is standard. Always inquire about penicillin allergy history before administration; anaphylaxis risk is ~0.04%. Use aqueous crystalline penicillin G for severe infections like meningitis (high CNS penetration with inflamed meninges). Monitor renal function in critically ill patients to adjust dosing. Pain at injection site is common with IM administration; consider using procaine penicillin G to reduce discomfort if appropriate. |
| Patient Advice | Complete the full course of medication even if you feel better. · Report any signs of allergic reaction (rash, itching, difficulty breathing, swelling) immediately. · This medication is given by injection; you may experience pain or redness at the injection site. · Inform your doctor if you have kidney disease or a history of allergies, especially to penicillins or cephalosporins. · Shake the vial well before use and use within the specified time after reconstitution. |