Penicillin G: veterinary dose and monograph

At a glance

Drug class

Antibiotic

Also known as

Procaine Penicillin G, Benzathine Penicillin G, Pen G, Crystalline Penicillin, Pfizerpen, Bicillin, APPG

Species with doses

Dogs, Cats, Cattle, Horses, Goats, Sheep, Pigs, Rabbits and Ferrets

Routes

IV, IM, PO, SC

Last reviewed

1 October 2026

Indications

• Streptococcal infections (strangles, mastitis, lymphadenitis)
• Clostridial myonecrosis / blackleg / tetanus / botulism
• Bovine respiratory disease complex (susceptibility-confirmed)
• Bovine / ovine foot rot (Fusobacterium)
• Leptospirosis (acute leptospiraemia in dogs and cattle)
• Actinomyces ("lumpy jaw" in cattle)
• Erysipelas in swine
• Anthrax (Bacillus anthracis)

Penicillin G dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

Dogs

• Bacteraemia / systemic infection (Pen G K): UNITS/kg. Plumb’s p.945 (Greene 2006): penicillin G potassium 20,000–40,000 U/kg IV q4–6h for bacteraemia/systemic infection (orthopaedic q6h; surgical prophylaxis 40,000 U/kg IV 1 h before). Papich 5e: potassium or sodium salt 20,000–40,000 U/kg q6–8h IV or IM. Use the sodium/potassium salt for IV — never procaine.
• Soft tissue / orthopaedic (procaine): UNITS/kg. Papich 5e and Plumb’s p.945 (Greene 2006): procaine penicillin G 20,000–40,000 U/kg IM or SC q12–24h; benzathine 40,000 U/kg IM every 5 days.
• Leptospiraemia (Greene 2006): UNITS/kg. Plumb’s p.945 (Ross & Rentko 2000): leptospiraemia 25,000–40,000 U/kg IV or IM q12–24h for 14 days, then doxycycline 5–10 mg/kg PO q12h for 14 days to clear the renal carrier state.

Cats

• Soft tissue infection (procaine): UNITS/kg. Plumb’s p.945 (Greene 2006): procaine penicillin G 20,000 U/kg IM or SC q12h; orthopaedic 20,000–40,000 U/kg IM q8h; benzathine 50,000 U/kg IM every 5 days. Papich 5e: 20,000–40,000 U/kg q12–24h IM.
• Actinomycosis / resistant streptococci: UNITS/kg. Plumb’s p.945 (Greene 2006): resistant organisms (Actinomyces) 50,000–100,000 U/kg procaine penicillin G IM or SC q12h.

Cattle

• Susceptible infection — procaine Pen G: Units/kg. Papich 5e: cattle and sheep, procaine penicillin G 22,000–66,000 U/kg q24h IM (SC discouraged). Plumb’s p.945: 25,000 U/kg once daily, twice daily for moderately susceptible bacteria (Gunn 2008); 40,000 U/kg IM daily (Howard 1986). These exceed the product label dose, so the label withdrawal does NOT apply — obtain a FARAD extended withdrawal. Rotate injection sites.
• BRD complex — high-dose extra-label: Units/kg. Plumb’s p.945 (Hjerpe 1986): procaine penicillin G 66,000 U/kg IM or SC once daily for bovine respiratory disease complex, with a recommended 20-day slaughter withdrawal at this dosage. Milk withdrawal is not given in the references — consult FARAD. Extra-label.
• Clostridial enteritis in calves (oral): UNITS/kg. Plumb’s p.945 (Callan & Rentko 2003): procaine penicillin G 10,000–20,000 U/kg PO q12–24h for 1–4 days for clostridial abomasitis/enteritis — oral penicillin is poorly absorbed and acts in the gut lumen. Extra-label — obtain a FARAD withdrawal.

Horses

• Procaine Pen G for susceptible infection: UNITS/kg. Plumb’s p.945: procaine penicillin G 22,000–44,000 U/kg IM q12h (Whittem); strangles 22,000 U/kg IM q12h (Foreman); foals 22,000–50,000 U/kg IM q12h. Papich 5e: 20,000–24,000 U/kg q24h IM. IM only — accidental intravascular procaine causes excitement/collapse.
• Severe / CNS infection (Pen G K or Na): UNITS/kg. Plumb’s p.945: serious Gram-positive infections (tetanus, botulism, C. difficile enterocolitis in foals) 22,000–44,000 U/kg IV q6h (Whittem; Johnston & Whitlock). Papich 5e: 20,000–24,000 U/kg q6–8h IV, up to 44,000 U/kg q6h for refractory cases.

Goats

• Susceptible infection (extra-label): UNITS/kg. Plumb’s p.945 (cattle and other ruminants): procaine penicillin G 25,000 U/kg once daily (twice daily for moderately susceptible bacteria); 10,000–40,000 U/kg IM q12–24h. Papich 5e (cattle and sheep): 22,000–66,000 U/kg q24h IM. No goat label — obtain a FARAD withdrawal for meat and milk.

Sheep

• Susceptible infection / foot rot (extra-label): UNITS/kg. Papich 5e (cattle and sheep): procaine penicillin G 22,000–66,000 U/kg q24h IM (SC discouraged). Plumb’s p.945 (ruminants): 10,000–40,000 U/kg IM q12–24h. Label withdrawals apply only to label doses — obtain FARAD advice.

Pigs

• Susceptible infection — procaine: Units/kg. Plumb’s p.945: 6,600 U/kg IM once daily for not more than 4 days (Wood 1986; the 7-day slaughter withdrawal applies to this label dose); 40,000 U/kg IM once daily (Howard 1986). Papich 5e: 15,000–25,000 U/kg q24h IM. Higher doses extend withdrawal — check FARAD.

Buffalo

No buffalo-specific doses are on file, so buffalo are dosed as cattle.

• Susceptible infection — procaine Pen G: Units/kg. Papich 5e: cattle and sheep, procaine penicillin G 22,000–66,000 U/kg q24h IM (SC discouraged). Plumb’s p.945: 25,000 U/kg once daily, twice daily for moderately susceptible bacteria (Gunn 2008); 40,000 U/kg IM daily (Howard 1986). These exceed the product label dose, so the label withdrawal does NOT apply — obtain a FARAD extended withdrawal. Rotate injection sites.
• BRD complex — high-dose extra-label: Units/kg. Plumb’s p.945 (Hjerpe 1986): procaine penicillin G 66,000 U/kg IM or SC once daily for bovine respiratory disease complex, with a recommended 20-day slaughter withdrawal at this dosage. Milk withdrawal is not given in the references — consult FARAD. Extra-label.
• Clostridial enteritis in calves (oral): UNITS/kg. Plumb’s p.945 (Callan & Rentko 2003): procaine penicillin G 10,000–20,000 U/kg PO q12–24h for 1–4 days for clostridial abomasitis/enteritis — oral penicillin is poorly absorbed and acts in the gut lumen. Extra-label — obtain a FARAD withdrawal.

Rabbits

• Venereal spirochetosis (Treponema) — PARENTERAL only: Plumb's p.946 (Ivey & Morrisey 2000): Procaine Pen G 20,000–84,000 U/kg SC/IM q24h ×5–7 days for rabbit syphilis (Treponema cuniculi). ⚠ ORAL penicillins are FATAL in rabbits (enterotoxaemia) — parenteral route ONLY, and monitor for dysbiosis.

Ferrets

• Susceptible infection: Plumb's p.946 (Williams 2000): Procaine Pen G 20,000–40,000 U/kg IM q12–24h; sodium/potassium Pen G 20,000 U/kg SC/IM/IV q4h, or 40,000 U/kg PO tid.

Contraindications

Known penicillin / cephalosporin hypersensitivity (cross-reactivity ~15% in humans). NEVER administer to rabbits, guinea pigs, chinchillas, hamsters — fatal clostridial enterotoxaemia; reptiles, birds (esp. small species), turtles also reportedly sensitive to procaine component. Septic / shocky animals: do not give oral pen G — switch to parenteral. High-dose Pen G K+ in renal failure / CHF can cause hyperkalaemia. Procaine penicillin: documented procaine reactions in horses (excitement, tachycardia, collapse) and racing-greyhound positives — withhold procaine forms before competition. Aqueous Pen G K+/Na+ contains 1.7 mEq K+ or 2 mEq Na+ per million units — adjust in electrolyte derangement.

Species-specific warnings

Horses: PROCAINE reactions can be fatal — aspirate before every IM injection, never give procaine forms IV. Withhold procaine forms before racing / competition (positive drug tests).

Cattle: Strict withdrawal: aqueous Pen G 4 d slaughter / 48 h milk; procaine 14 d / 48 h. Mastitis intramammary syringes: per product label.

Rabbits: NEVER use — fatal Clostridium dysbiosis even with parenteral route.

Small mammals: NEVER in guinea pigs, hamsters, chinchillas, gerbils — fatal dysbiosis.

Dogs: Standard for streptococcal / anaerobic infections. Watch for hypersensitivity. K-Pen G caution in renal failure (hyperkalaemia).

Adverse effects

• Type-I hypersensitivity / anaphylaxis (rare but classical)
• Procaine reactions in horses — excitement, tachycardia, collapse minutes after IM injection
• Injection-site swelling / sterile abscess (procaine forms)
• GI upset with oral forms (PO bioavailability poor; food further reduces it)
• Antibiotic-associated diarrhoea / colitis in susceptible species
• Electrolyte disturbance (hyperkalaemia / hypernatraemia) with high-dose IV

Drug interactions

• Aminoglycosides (gentamicin, streptomycin): SYNERGISTIC against Enterococcus, Listeria, Streptococcus — first-line combo for these. DO NOT mix in syringe (inactivation).
• Bacteriostatic antibiotics (tetracyclines, chloramphenicol, macrolides): theoretical antagonism
• Probenecid: blocks tubular secretion — prolongs penicillin half-life (clinically used historically)
• Warfarin: may potentiate anticoagulation
• Methotrexate: decreased renal clearance — toxicity risk
• Procaine penicillin: contains procaine LA — caution with other procaine-sensitive patients

Pregnancy and lactation

Generally considered safe in pregnant and lactating animals at therapeutic doses. Crosses placenta and enters milk in low concentrations. FDA Category B (human). Widely used in pregnant mares (strangles, leptospirosis prevention) and dairy cattle (intramammary).

Monitoring

• Hypersensitivity signs — especially first dose
• Withdrawal periods strictly: cattle aqueous Pen G 4 d slaughter / 48 h milk; procaine Pen G 14 d / 48 h; benzathine 30 d
• Horses: aspirate before IM procaine injection — verify no blood return (procaine = sudden death risk if IV)
• Renal function on prolonged high-dose courses
• Electrolytes (K+) with K-Pen G in renal / cardiac patients
• Injection site for sterile abscess (procaine) — rotate sites

Toxicity and overdose

Procaine penicillin: PROCAINE REACTIONS in horses (excitement, ataxia, collapse, sudden death within minutes of IM injection) — caused by accidental intravenous injection or rapid systemic absorption. Self-injection or accidental IV: anaphylactoid reaction in handler. Hypersensitivity (urticaria, anaphylaxis) — cross-reactivity with cephalosporins ~10%. Pen G K+ in CHF / renal failure: hyperkalaemia (each million units = 1.7 mEq K+). Lagomorphs / small mammals: fatal Clostridium dysbiosis even with parenteral route.

Management: Procaine reaction: stop injection, administer diazepam 0.25–0.5 mg/kg IV for excitement, support respiration. Anaphylaxis: epinephrine 0.01 mg/kg IM (1:1000), diphenhydramine, corticosteroid, IV fluids. Hyperkalaemia from massive IV K+: calcium gluconate IV, insulin + glucose, sodium bicarbonate. No specific antidote — supportive.

Clinical pearls

• Streptococcus equi (strangles): 22,000 IU/kg IM procaine pen q12h × 5–10 days — first-line for treatment and post-exposure prophylaxis
• Bovine clostridial myonecrosis / blackleg: 22,000–44,000 IU/kg IM procaine q12h × 5 days
• Leptospirosis treatment: 25,000–40,000 IU/kg IM/IV q12h × 14 days (then doxycycline for renal shedding clearance)
• Aspirate before IM procaine injection in horses — confirm needle is NOT intravenous
• NEVER use in lagomorphs, guinea pigs, chinchillas, hamsters — fatal Clostridium overgrowth EVEN with parenteral route
• Long-acting benzathine Pen G is OFTEN MISUSED — only adequate blood levels for highly susceptible Streptococcus; insufficient for most other organisms
• Tetanus / botulism: high-dose Pen G + antitoxin — Pen G kills C. tetani at the wound site

Mechanism of action

Natural beta-lactam antibiotic produced by Penicillium. Binds penicillin-binding proteins (PBPs), inhibiting transpeptidase cross-linking of peptidoglycan. Bactericidal, time-dependent. Spectrum: streptococci, anaerobes (Clostridium except C. difficile, Fusobacterium, Actinomyces), spirochaetes (Leptospira, Treponema, Borrelia), some Gram-positive aerobes. Inactivated by Staphylococcus aureus beta-lactamase. Short half-life — q4–6h aqueous, depot procaine q12–24h, depot benzathine q3–5 d (only for highly susceptible Streptococcus).

Formulations and products

• Procaine Pen G suspension 300,000 U/mL
• Procaine Pen G / Benzathine Pen G combo 150,000 U/mL each
• Penicillin G potassium 1,000,000 U dry-powder vial — reconstitute per label (dosed in IU)
• Penicillin G sodium 5,000,000 U dry-powder vial — reconstitute per label (dosed in IU; Plumb’s p.947)

Storage

Crystalline / aqueous powders: refrigerate after reconstitution, use within 24 h. Procaine penicillin: refrigerate, shake well before use. Benzathine: refrigerate; viscous — warm before injection. Multidose vials: discard 28 days after broaching.

Before you use this dose

Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

Other antibiotics

• Norfloxacin
• Orbifloxacin
• Ornidazole
• Oxytetracycline
• Pirlimycin
• Polymyxin B
• Pradofloxacin
• Procaine Penicillin G

Frequently asked questions