DogSource: Greene 2006
Bacteremia / systemic infection: 20,000–40,000 units/kg · every 4–6 h
Antibiotic Therapy
Penicillin G
Natural (injectable) penicillin · bactericidal (cell-wall synthesis inhibitor)
Species: Dogs & Cats13 dose protocols7 dosage forms
This page is a calculation and educational reference for veterinarians and veterinary students. It does not replace examination, culture and susceptibility testing, clinical judgment, or the attending veterinarian's final decision.
Drug overview
Natural (injectable) penicillin · bactericidal (cell-wall synthesis inhibitor)
Brand names: Pfizerpen®, Bicillin®
General aqueous (potassium) dose: 20,000–40,000 units/kg parenteral · every 4–6 hSource: Greene 2006 (aqueous/systemic)
Spectrum of activity
A natural, bactericidal penicillin with the narrowest spectrum among the penicillins. It is active against susceptible Gram-positive cocci (streptococci, enterococci), Gram-positive bacilli (Listeria, Clostridium, Actinomyces, Bacillus anthracis), many anaerobes (except Bacteroides fragilis), the Gram-negative coccus Neisseria, Pasteurella, and spirochetes (Leptospira, Borrelia, Treponema). But unlike the aminopenicillins it does not cover the enteric Gram-negatives (E. coli, Klebsiella) or Haemophilus; and because it has no beta-lactamase inhibitor, it is inactive against penicillinase-producing staphylococci (most staph). It also has no effect on Pseudomonas, Mycoplasma, Rickettsia or Mycobacterium.
Veterinary uses and doses
Aqueous — potassium (fast-acting, IV/IM)
DogSource: Greene 2006
Orthopedic infections: 40,000 units/kg IV · every 6 h
DogSource: Greene 2006
Orthopedic surgical prophylaxis: 40,000 units/kg IV · one hour before surgery
DogSource: Ross 2000
Leptospirosis (leptospiremic phase): 25,000–40,000 units/kg · every 12–24 h · 14 days
Clinical note: Penicillin G covers only the acute leptospiremic phase and does not clear the renal carrier state; to clear the renal carrier, follow this course with doxycycline 5–10 mg/kg twice daily for 14 days.
CatSource: Aronson 1989
Cat: 20,000–40,000 units/kg IV or IM · every 6 h
Procaine — 400k/800k/1M (once daily, IM)
DogSource: Greene 2006
General infection: 20,000–40,000 units/kg IM · every 12–24 h
DogSource: Ford 1985
Actinomycosis: 100,000–200,000 units/kg IM · once daily
CatSource: Kirk 1989
Cat: 20,000 units/kg IM or SC · every 12–24 h
CatSource: Greene 2006
Cat (Actinomyces): 50,000–100,000 units/kg IM · every 12 h
6.3.3 (combination, once daily, IM)
DogSource: based on the procaine component
General susceptible infection: 20,000–40,000 units/kg IM · once daily
CatSource: based on the procaine component
Cat (general susceptible infection): 20,000–40,000 units/kg IM · once daily
Benzathine (long-acting, IM)
DogSource: Greene 2006
40,000 units/kg IM · every 5 days
CatSource: Greene 2006
Cat: 50,000 units/kg IM · every 5 days
Dosage forms
- Penicillin G potassium 1,000,000 units
- Penicillin G potassium 5,000,000 units
- Procaine penicillin G 400,000 units
- Procaine penicillin G 800,000 units
- Procaine penicillin G 1,000,000 units
- 6.3.3 — 1,200,000 units (600 procaine + 300 benzathine + 300 potassium)
- Benzathine penicillin G 1,200,000 units
Safety and clinical notes
- IM injection of penicillin G potassium may cause severe pain and tissue irritation; IV is the first choice.
- Never give benzathine or procaine forms intravenously (IM only).
- Contraindicated in penicillin allergy; cross-reactivity with cephalosporins and carbapenems is possible.
- Never give to rabbits, guinea pigs, chinchillas or hamsters — risk of fatal enteritis (it is safe in dogs and cats). Some species (reptiles, birds, guinea pigs) are sensitive to procaine penicillin G.
- High-dose penicillin G sodium/potassium in a small animal with renal or cardiac failure can cause a sodium/potassium load and electrolyte imbalance; overly rapid IV injection may cause neurologic signs.
- Do not mix with aminoglycosides (amikacin, gentamicin) in one syringe or solution (incompatible); compatible with metronidazole.
- Considered relatively safe in pregnancy (class A in dogs and cats).
Cited sources
- Greene 2006 (aqueous/systemic)
- Greene 2006
- Ross 2000
- Aronson 1989
- Ford 1985
- Kirk 1989
- based on the procaine component
Calculate a weight-based dose
Drug-data last updated: