Penicillin G

Penicillin G, or benzylpenicillin, is a natural beta-lactam antibacterial used against susceptible organisms. Soluble sodium or potassium salts provide rapid systemic exposure, while procaine and benzathine formulations prolong release. Veterinary applications emphasize susceptible streptococci, selected gram-positive anaerobes, spirochetes and a limited group of other bacteria.

Group

Beta-lactam antibacterial

Mechanism of Action

Penicillin G binds bacterial penicillin-binding proteins, including transpeptidases involved in peptidoglycan cross-linking. Inhibition of cell-wall assembly weakens growing bacteria and leads to bactericidal activity. The effect is time dependent and relates to exposure above the organism’s inhibitory concentration. Different salts alter release and exposure but supply the same antibacterial penicillin molecule.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Susceptible infection with soluble penicillin G	20000–40000 IU/kg	IV, IM	Every 6–8 h	Sodium or potassium salt only.
Susceptible infection with procaine penicillin G	20000–40000 IU/kg	IM	Every 12–24 h	Procaine suspension; not for IV administration.
Bacteremia with soluble penicillin G	20000–40000 IU/kg	IV	Every 4–6 h	Potassium salt.
Orthopedic infection with soluble penicillin G	20000–40000 IU/kg	IV	Every 6 h	Potassium salt.
Soft tissue infection with soluble penicillin G	40000–60000 IU/kg	PO	Every 8 h	Historical potassium-salt oral regimen; oral exposure is poor.
Leptospiremia	25000–40000 IU/kg	IV, IM	Every 12–24 h	Up to 14 days; 14 days; subsequent doxycycline addresses renal carriage.
Orthopedic surgery prophylaxis	40000 IU/kg	IV	Once	Potassium salt 1 hour preoperatively; repeat if surgery exceeds 90 minutes.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Susceptible infection with soluble penicillin G	20000–40000 IU/kg	IV, IM	Every 6–8 h	Sodium or potassium salt only.
Susceptible infection with procaine penicillin G	20000–40000 IU/kg	IM	Every 12–24 h	Procaine suspension; not for IV administration.
Soft tissue infection with soluble penicillin G	40000 IU/kg	PO	Every 6–8 h	Historical potassium-salt oral regimen; oral exposure is poor.
Soft tissue infection with procaine penicillin G	20000 IU/kg	IM	Every 12 h
Orthopedic infection with procaine penicillin G	20000–40000 IU/kg	IM	Every 8 h
Actinomyces infection with procaine penicillin G	50000–100000 IU/kg	IM	Every 12 h

Cattle

Use / stage	Dose	Route	Schedule	Instructions
Susceptible infection with soluble penicillin G	20000 IU/kg	IV, IM	Every 6 h	Sodium or potassium salt only.
Susceptible infection with procaine penicillin G	22000–66000 IU/kg	IM	Every 24 h	Procaine suspension; not for IV administration.
Clostridial enteritis in calves	10000–20000 IU/kg	PO	Every 12–24 h	Up to 4 days; Procaine penicillin G orally for local intestinal action, 1-4 days; not for systemic infection.
Respiratory disease with procaine penicillin G	66000 IU/kg	IM	Every 24 h
Susceptible infection with procaine penicillin G	40000 IU/kg	IM	Every 24 h

Sheep

Use / stage	Dose	Route	Schedule	Instructions
Susceptible infection with soluble penicillin G	20000 IU/kg	IV, IM	Every 6 h	Sodium or potassium salt only.
Susceptible infection with procaine penicillin G	22000–66000 IU/kg	IM	Every 24 h	Procaine suspension; not for IV administration.

Pigs

Use / stage	Dose	Route	Schedule	Instructions
Susceptible infection with procaine penicillin G	15000–25000 IU/kg	IM	Every 24 h	Procaine suspension; not for IV administration.
Susceptible infection with procaine penicillin G	40000 IU/kg	IM	Every 24 h
Susceptible infection with procaine penicillin G	6600 IU/kg	IM	Every 24 h	Up to 4 days

Horses

Use / stage	Dose	Route	Schedule	Instructions
Susceptible infection with procaine penicillin G	20000–24000 IU/kg	IM	Every 24 h	Procaine suspension; not for IV administration.
Susceptible infection with soluble penicillin G	20000–24000 IU/kg	IV	Every 6–8 h	Sodium or potassium salt.
Refractory infection with soluble penicillin G	44000 IU/kg	IV	Every 6 h	Higher reported regimen for refractory cases.
Susceptible infection with soluble penicillin G	20000–24000 IU/kg	IM	Every 12 h	Potassium salt only.
Sodium penicillin infusion	Reference only	IV		The reported guidance gives 1.3 mg/kg loading followed by 2.5 mcg/kg/hour to target 2 mcg/mL. The stated maintenance rate and target concentration require clarification before calculation.
Gram-positive infection with soluble penicillin G	10000–20000 IU/kg	IV, IM	Every 6 h	Sodium or potassium salt.
Serious gram-positive infection	22000–44000 IU/kg	IV	Every 6 h	Sodium or potassium salt; includes tetanus/botulism reports.
Susceptible infection with procaine penicillin G	22000–44000 IU/kg	IM	Every 12 h	Procaine suspension.
Foal infection with soluble penicillin G	20000–50000 IU/kg	IV	Every 6–8 h	Foals; sodium or potassium salt.
Foal infection with procaine penicillin G	22000–50000 IU/kg	IM	Every 12 h	Foals; procaine suspension.
Foal infection with procaine penicillin G	20000–50000 IU/kg	IM	Every 6 h	Separate intensive foal regimen.

Ferrets

Use / stage	Dose	Route	Schedule
Susceptible infection with procaine penicillin G	20000–40000 IU/kg	IM	Every 12–24 h
Susceptible infection with soluble penicillin G	20000 IU/kg	SC, IM, IV	Every 4 h
Susceptible infection with soluble penicillin G	40000 IU/kg	PO	Every 8 h

Rabbits

Use / stage	Dose	Route	Schedule	Instructions
Venereal spirochetosis	20000–84000 IU/kg	SC, IM	Every 24 h	Up to 7 days; Procaine form; reported 5-7 day course.

Hedgehog

Use / stage	Dose	Route	Schedule	Instructions
Susceptible infection	40000 IU/kg	IM	Every 24 h

Indications

Treatment of susceptible streptococcal respiratory, soft-tissue or systemic infections in veterinary patients.
Treatment of selected susceptible anaerobic infections, including some clostridial and Actinomyces infections.
Treatment of selected susceptible bone or joint infections.
Initial treatment of leptospiremia within a complete leptospirosis treatment plan.
Treatment of selected susceptible equine or livestock infections with a formulation appropriate to the infection site.
Treatment of rabbit treponematosis with a supported parenteral penicillin regimen.

Contraindications

Penicillin hypersensitivity
Never administer procaine or benzathine suspensions intravenously.

Adverse effects

Allergic reactions, injection injury and procaine-associated CNS reactions in horses; sodium loading and potassium changes with high doses.

Toxicity

Very high parenteral penicillin exposure, especially with impaired renal elimination, can cause CNS toxicity; oral excess more often causes GI upset.

Preparations

Preparation	Form	Strength	Route
Penicillin G potassium 5000000 IU vial	Injection	5000000 IU / vial	IV, IM
Penicillin G potassium 20000000 IU vial	Injection	20000000 IU / vial	IV, IM
Procaine penicillin G 300000 IU/mL	Injection	300000 IU / mL	IM

Monitoring and Administration

Select soluble sodium/potassium salts for IV use and procaine suspension for appropriate IM use. Avoid procaine SC injection and injection volumes above 30 mL/site. Benzathine concentrations are generally too low for most infections.

Refrigerate reconstituted soluble salts: reported experience indicates 7-day stability, with 90% potency at 14 days. Replace solutions prepared for continuous infusion every 12 hours; avoid strong acid, alkaline pH above 8 and oxidizers.

Equine S. equi guttural-pouch protocol: after removing inflammatory material, dissolve 2 g gelatin in 40 mL sterile water, cool to 45-50 C, combine with 10 million IU sodium penicillin G reconstituted in 10 mL, giving 50 mL final gel. Refrigerate overnight, instill by endoscopically guided catheter and elevate the head for 20 minutes. This is a local procedure, not a systemic injection.

Additional benzathine/procaine combinations and isolated benzathine regimens produce prolonged low exposure and are distinct from soluble penicillin. Older references include procaine SC use, conflicting with tissue-injury and residue cautions; do not silently substitute SC for IM. For strangles, treatment decisions depend on disease stage and systemic illness.