Penicillin G (Pfizerpen) — Compare Prices at US & Canada Pharmacies

Penicillin G potassium and sodium (brand Pfizerpen) are the aqueous, fast-acting forms of penicillin G — distinct from the long-acting benzathine and procaine salts (like Bicillin L-A) that are built for slow, sustained release. Because it's given IV or IM and reaches high blood levels quickly, this form is used for serious, rapidly progressing infections that need immediate high-dose penicillin: bacteremia, endocarditis, meningitis, pneumonia, and severe skin or soft-tissue infections caused by penicillin-susceptible organisms, plus specific indications like neurosyphilis, anthrax, and clostridial infections. That's the opposite clinical role from benzathine penicillin G, which is deliberately released slowly over weeks and is never given IV — an important distinction, since injecting the benzathine formulation intravenously has caused fatal cardiorespiratory arrest, a risk that doesn't apply to this aqueous IV/IM form.

Penicillin G potassium specifically carries a notable dose-dependent risk: at very high continuous IV doses, especially in patients with reduced kidney function, the potassium load from the drug itself can raise blood potassium to dangerous levels, occasionally causing cardiac arrhythmias. This is one reason the sodium salt is sometimes preferred in patients where potassium accumulation is a concern, and why electrolyte monitoring is routine during high-dose IV therapy.

Two other clinically important issues apply broadly to penicillin G: hypersensitivity reactions, ranging from rash to life-threatening anaphylaxis, occur in a meaningful minority of patients, so allergy history is always checked before starting therapy; and when treating spirochete infections like syphilis, a Jarisch-Herxheimer reaction — fever, chills, and a transient drop in blood pressure caused by the rapid die-off of bacteria releasing endotoxin-like substances — can occur within hours of the first dose and is a recognized, usually self-limited, reaction rather than a drug allergy.

Boxed warning

WARNING NOT FOR INTRAVENOUS USE. DO NOT INJECT INTRAVENOUSLY OR ADMIX WITH OTHER INTRAVENOUS SOLUTIONS. THERE HAVE BEEN REPORTS OF INADVERTENT INTRAVENOUS ADMINISTRATION OF PENICILLIN G BENZATHINE WHICH HAS BEEN ASSOCIATED WITH CARDIORESPIRATORY ARREST AND DEATH. Prior to administration of this drug, carefully read the WARNINGS , ADVERSE REACTIONS , and DOSAGE AND ADMINISTRATION sections of the labeling.

Contraindications

CONTRAINDICATIONS A history of a previous hypersensitivity reaction to any of the penicillins is a contraindication.

Warnings

WARNINGS WARNING NOT FOR INTRAVENOUS USE. DO NOT INJECT INTRAVENOUSLY OR ADMIX WITH OTHER INTRAVENOUS SOLUTIONS. THERE HAVE BEEN REPORTS OF INADVERTENT INTRAVENOUS ADMINISTRATION OF PENICILLIN G BENZATHINE WHICH HAS BEEN ASSOCIATED WITH CARDIORESPIRATORY ARREST AND DEATH. Prior to administration of this drug, carefully read the WARNINGS , ADVERSE REACTIONS , and DOSAGE AND ADMINISTRATION sections of the labeling. Penicillin G benzathine should only be prescribed for the indications listed in this insert. Anaphylaxis SERIOUS AND OCCASIONALLY FATAL HYPERSENSITIVITY (ANAPHYLACTIC) REACTIONS HAVE BEEN REPORTED IN PATIENTS ON PENICILLIN THERAPY.

Pregnancy & breastfeeding

Pregnancy Teratogenic effects Pregnancy Reproduction studies performed in the mouse, rat, and rabbit have revealed no evidence of impaired fertility or harm to the fetus due to penicillin G. Human experience with the penicillins during pregnancy has not shown any positive evidence of adverse effects on the fetus. There are, however, no adequate and well-controlled studies in pregnant women showing conclusively that harmful effects of these drugs on the fetus can be excluded.