Pen G: Penicillin G Use in Veterinary Medicine

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

Pen G is the veterinary shorthand for penicillin G, also called benzylpenicillin. It is a narrow-spectrum beta-lactam antibiotic that kills susceptible gram-positive bacteria by disrupting the bacterial cell wall. In veterinary practice it is sold as three main injectable salts, aqueous sodium or potassium penicillin G (short-acting), procaine penicillin G (intermediate-acting), and benzathine penicillin G (long-acting), and it is used chiefly in cattle, horses, swine, and small animals for infections caused by organisms such as Streptococcus species, Actinomyces pyogenes, and susceptible Staphylococcus species, plus a small number of gram-negative and anaerobic organisms. It is a prescription drug in every formulation and every species. It is not a treatment for viral infections, it does not cover most gram-negative bacteria or any beta-lactamase producer, and it can be fatal in rabbits, guinea pigs, and hamsters because of disruption of the hindgut microbiome.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

At a Glance

What Penicillin G Is and How It Works

Penicillin G is a natural beta-lactam antibiotic produced by Penicillium molds. Its core structure is a four-membered beta-lactam ring fused to a thiazolidine ring. That beta-lactam ring is the warhead. It binds irreversibly to bacterial penicillin-binding proteins, which are the enzymes that cross-link peptidoglycan strands in the growing cell wall. When cross-linking stops, the wall weakens, the bacterium takes on water, and it lyses. Because mammalian cells have no peptidoglycan wall, the drug is selectively toxic to bacteria, which is why its dose-limiting toxicity in animals is usually local irritation or neurologic signs at very high blood concentrations rather than organ damage.

Penicillin G is bactericidal and time-dependent. Its killing depends on how long the free drug concentration stays above the minimum inhibitory concentration (MIC) of the target organism, not on how high the peak goes. That single pharmacokinetic fact explains the entire formulation strategy. Aqueous sodium or potassium penicillin G produces a high peak that falls quickly, so it must be dosed frequently. Procaine penicillin G is a poorly soluble salt that slowly releases free drug from the injection site, giving roughly a 24-hour window. Benzathine penicillin G is even less soluble and releases drug over several days. Repository salts trade peak concentration for duration, which is exactly what a time-dependent killer wants.

The drug is distributed mainly into extracellular fluid. It penetrates inflamed meninges, synovial fluid, and pleural fluid reasonably well, but it does not enter cells effectively and it does not cross intact barriers well. It is eliminated largely unchanged by renal tubular secretion, which is fast in most species. Probenecid blocks that tubular secretion and prolongs blood concentrations, a trick that is used in some compounded products and in human medicine. In food animals, penicillin G is one of the most frequently detected violative residues, and it has among the highest potentials of any antibiotic to provoke allergic reactions in people who eat contaminated meat or milk [1]. That is why the withdrawal time on the label is not a suggestion.

The Three Injectable Formulations Compared

Aqueous Sodium or Potassium Penicillin G

This is the short-acting form. It is highly water-soluble, so it is absorbed quickly after intramuscular or subcutaneous injection and can also be given intravenously. Blood concentrations peak within minutes to a couple of hours and fall with a half-life measured in hours or less, so dosing is typically every 4 to 6 hours in small animals and every 6 to 12 hours in large animals depending on the label. It is the formulation to reach for when a high immediate concentration is needed, for example in a severe Streptococcus equi infection in a horse or a rapidly progressing soft tissue infection in a dog. It is also the form used for intravenous therapy in horses and for intrauterine or intramammary infusion when a product is labeled for that route.

Procaine Penicillin G

Procaine penicillin G is a salt of penicillin G with procaine. The procaine portion limits how fast the salt dissolves in tissue fluid, so free penicillin G is released slowly. A single intramuscular dose typically maintains therapeutic concentrations for about 24 hours in cattle and horses, which is why most cattle products are labeled for once-daily injection. Peak concentrations are lower than with the aqueous salt, so procaine penicillin G is a good choice for susceptible infections that do not need an immediate high peak, such as routine respiratory disease or foot rot in cattle. It should not be given intravenously, and in horses it is given intramuscularly only. Procaine itself can cause excitement or neurologic signs if it is accidentally injected intravascularly, which is why the intramuscular route and correct needle placement matter.

Benzathine Penicillin G

Benzathine penicillin G is the longest-acting salt. It is very poorly soluble and releases drug over days. It is used when prolonged low-level exposure is the goal, for example in some equine respiratory infections or in combination products where a small amount of procaine penicillin G provides an early peak and benzathine penicillin G provides the tail. It is not a substitute for frequent dosing when a high concentration is needed, because the concentrations it produces are low. Combination procaine/benzathine products are common in small animal and equine practice, and their behavior is a blend of both salts.

Formulation Comparison Table

Withdrawal times differ by formulation, species, route, and country, and they are printed on the approved label for the exact product being used. A procaine penicillin G product labeled for cattle may carry a different meat withdrawal time than a combination procaine/benzathine product labeled for the same species, and the milk withdrawal time for an intramammary product is different again. Always read the label of the bottle in your hand. Do not transfer a withdrawal time from one product to another, and do not transfer a withdrawal time from one country to another. The label is the legal and clinical authority.

Labeled Uses and What Penicillin G Does Not Cover

Penicillin G is labeled for the treatment of infections caused by susceptible gram-positive bacteria. In cattle, common labeled indications include bovine respiratory disease complex caused by susceptible Pasteurella and Streptococcus organisms, foot rot, and mastitis when an intramammary product is used. In horses, it is used for strangles and other Streptococcus equi infections, for Actinomyces pyogenes infections, and for many soft tissue infections. In swine, it is used for erysipelas and for respiratory and soft tissue infections caused by susceptible organisms. In dogs and cats, it is used for susceptible skin, soft tissue, urinary, and respiratory infections, though amoxicillin and other oral beta-lactams are more common in small animal practice because of convenience.

Penicillin G does not cover Escherichia coli, Klebsiella, Pseudomonas, Proteus, or most other Enterobacterales, because those organisms produce beta-lactamases or have impermeable outer membranes. It does not cover Mycoplasma species, which lack a cell wall and are therefore intrinsically resistant to every beta-lactam. It does not cover methicillin-resistant Staphylococcus aureus (MRSA) or other methicillin-resistant staphylococci. It does not cover Bordetella bronchiseptica, and it does not cover fungi, viruses, or parasites. In dairy cattle, penicillin G resistance in Staphylococcus aureus isolated from mastitis milk is well documented, with high resistance rates reported in field surveys [2]. That means culture and susceptibility testing is not optional when a cow has mastitis that is not responding, and it is the reason blind use of penicillin G for every mastitis case is poor practice.

Dosing Principles and a Species Dose Table

Doses must come from the approved label for the product in hand or from a current veterinary formulary. Doses vary by species, by formulation, by route, and by the target organism. The numbers below are representative label or formulary values for orientation only. They are not a substitute for the label, and they are not a substitute for a veterinarian's prescription.

The guinea pig and chinchilla figure comes from a retrospective study of 58 animals treated with parenteral penicillin G procaine/benzathine at a veterinary teaching hospital, where the median dose was 50,000 IU/kg subcutaneously [3]. That study reported suspected adverse effects in 1 of 15 guinea pigs and 2 of 43 chinchillas, and no gastrointestinal signs such as diarrhea were recorded as adverse effects. That finding is interesting but it does not overturn the long-standing rule that penicillins are dangerous in these species. The study is retrospective, the numbers are small, and adverse effects are hard to separate from the underlying disease. The standard teaching remains that penicillins and other antibiotics that reach the hindgut should be avoided in rabbits, guinea pigs, and hamsters.

How Penicillin G Is Given

Intramuscular injection is the most common route in large animals. Use a clean needle, a fresh injection site, and a needle long enough to reach muscle in cattle and horses. Rotate sites to avoid muscle damage and abscesses. Procaine penicillin G should never be given intravenously, because the procaine can cause neurologic signs. Aqueous sodium or potassium penicillin G can be given intravenously, but it should be injected slowly and the animal should be observed.

Subcutaneous injection is used in small animals and in some large animal products. It is generally well tolerated but absorption may be slower and less predictable than from muscle.

Intramammary infusion is used for mastitis. The teat end must be cleaned and disinfected, the cannula must be inserted only partway to avoid contaminating the teat cistern, and the product must be massaged up into the quarter. Milk from treated quarters must be discarded for the full labeled milk withdrawal period.

Oral administration is used in unweaned calves, where penicillin G is sometimes fed in milk or milk replacer. Oral absorption is poor. In one study of 1-week-old and 5-week-old calves given 4.0 mg/kg orally, bioavailability was only 10.2% at 1 week and 7.4% at 5 weeks, and the maximum plasma concentration was 85 ng/mL at 1 week and 58 ng/mL at 5 weeks [4]. That is far below what is needed to treat most systemic infections, and feeding milk containing penicillin G to calves is a recognized cause of violative liver residues [5]. Oral penicillin G in calves is a residue risk more than a therapy.

Side Effects and What to Do About Them

The most common adverse effects of penicillin G in animals are pain and swelling at the injection site, especially with procaine and benzathine salts, and especially with large volumes or repeated injections into the same site. Muscle fibrosis and abscesses can follow. Rotating sites and using the smallest effective volume reduce this.

Hypersensitivity reactions are uncommon but real. They range from urticaria and facial swelling to anaphylaxis. Anaphylaxis requires immediate epinephrine, intravenous fluids, and supportive care. If an animal has had a reaction to a penicillin, do not give another penicillin.

Procaine toxicity can occur if procaine penicillin G is accidentally injected into a blood vessel. Signs include excitement, muscle tremors, ataxia, and in severe cases seizures. Treatment is supportive.

Very high blood concentrations of penicillin G, usually from intravenous overdose or from renal failure that prevents elimination, can cause neurologic signs including tremors and seizures. This is more likely in animals with kidney disease, so dose adjustment or a different drug is appropriate in those patients.

In horses, intramuscular procaine penicillin G can cause local reactions and, rarely, signs attributed to procaine. In small animals, oral penicillins can cause vomiting and diarrhea, though oral penicillin G is not commonly used in dogs and cats.

In rabbits, guinea pigs, and hamsters, the most serious adverse effect is not an allergy or an injection reaction. It is fatal dysbiosis. These species are hindgut fermenters that depend on a dense population of gram-positive bacteria in the cecum and colon to ferment fiber and produce volatile fatty acids. Penicillin G given systemically reaches the gut and kills those bacteria. The result is a shift toward gram-negative organisms, including Clostridium species, and a syndrome of severe diarrhea, toxemia, and death that can occur within days. This is why penicillins, including amoxicillin and ampicillin, are contraindicated in these species. If a rabbit or guinea pig has been given penicillin G by mistake, contact a veterinarian immediately. Treatment usually involves aggressive fluid therapy, a prokinetic, and sometimes cholestyramine to bind the toxin, and it must be started before diarrhea appears if possible.

Which Animals Should Not Receive Penicillin G

Rabbits, guinea pigs, hamsters, and other small hindgut fermenters should not receive penicillin G or any other penicillin or cephalosporin by any route. The risk of fatal dysbiosis is high and the benefit is almost never worth it. There are safer choices for these species, including fluoroquinolones, trimethoprim-sulfonamides, and some tetracyclines, chosen with culture results and veterinary guidance.

Animals with a known penicillin allergy should not receive it.

Animals with severe renal failure should receive it with caution, because elimination is reduced and neurologic toxicity becomes more likely.

Animals that are intended for food production must be treated only with a product labeled for that species and that production class, and the labeled withdrawal time must be observed. Extralabel use in food animals is a food safety issue and in many countries is restricted by law.

Drug Interactions

Probenecid blocks renal tubular secretion of penicillin G and prolongs its blood concentrations. This interaction is used deliberately in some compounded products.

Aminoglycosides such as gentamicin and streptomycin are often combined with penicillin G because the combination is synergistic against some organisms, including Streptococcus species and enterococci. The clinical pharmacology of penicillin G and dihydrostreptomycin or streptomycin combinations in dairy cattle has been reviewed, and the combination is marketed for parenteral and intramammary use in some countries [6]. The synergy is real for some organisms and not for others, and the combination does not extend coverage to beta-lactamase producers.

Bacteriostatic antibiotics such as tetracyclines and chloramphenicol can theoretically antagonize the bactericidal effect of penicillin G, because penicillin G kills actively dividing bacteria and bacteriostatic drugs stop division. In practice this interaction is debated and is rarely the deciding factor in a clinical case, but it is a reason to avoid unnecessary combinations.

Do not mix penicillin G with aminoglycosides or tetracyclines in the same syringe, because chemical incompatibility can inactivate one or both drugs. Give them at separate sites.

How Penicillin G Compares With Alternatives

Amoxicillin is an oral aminopenicillin with better absorption and a broader spectrum than penicillin G. It is more convenient in small animals because it can be given by mouth. It is still a beta-lactam and still ineffective against beta-lactamase producers unless it is combined with clavulanic acid.

Ampicillin is an injectable aminopenicillin with a spectrum similar to amoxicillin. It is used in large animals and small animals when an injectable beta-lactam is needed.

Ceftiofur is a third-generation cephalosporin used widely in cattle, swine, and horses. It has a broader gram-negative spectrum than penicillin G and is stable to many beta-lactamases. In a retrospective study of guinea pigs and chinchillas, parenteral ceftiofur crystalline free acid was administered to 42 guinea pigs and 24 chinchillas with no adverse effects reported, in contrast to the suspected adverse effects in a small number of animals given penicillin G procaine/benzathine [3]. That does not make ceftiofur a routine choice in these species, but it does illustrate that not all beta-lactams carry the same risk and that species-specific data matter.

Oxytetracycline is a broad-spectrum bacteriostatic antibiotic used in cattle and other species. It is not a substitute for penicillin G when the target is a susceptible Streptococcus, but it covers some organisms that penicillin G does not.

Florfenicol is a broad-spectrum bacteriostatic antibiotic used in cattle and swine. It is not a beta-lactam and is not affected by beta-lactamases.

The choice between penicillin G and any alternative should be based on the organism, the site of infection, the species, the route available, the withdrawal time, and the cost. Culture and susceptibility testing is the most reliable way to make that choice when the infection is serious or when previous treatment has failed.

Residues and Withdrawal Times in Food Animals

Penicillin G is one of the most frequently detected violative residues in food-producing animals, and it is one of the antibiotics most likely to cause allergic reactions in consumers [1]. The FDA-approved label withdrawal time is the legal minimum for the product, species, and route on that label. It is not a general rule that can be applied to other products.

Physiologically based pharmacokinetic (PBPK) models have been used to predict tissue residues and withdrawal intervals for penicillin G in cattle and swine. In one study, predicted withdrawal intervals after extralabel administration were close to or slightly longer than the FDA-approved label times, with a predicted 7 days versus a labeled 4 days for penicillin G in cattle [7]. In another PBPK study, extralabel use at 5 times and 10 times the label dose required longer withdrawal intervals to bring tissue concentrations below tolerance for the 99th percentile of the population [8]. A separate PBPK model for milk predicted a milk discard interval of 182 hours after 3 repeated intramuscular doses at 10 times the label dose, and 122 hours after 3 repeated intramammary infusions at 4 times the label dose [9]. These modeled intervals are longer than the label and illustrate why extralabel use in dairy cattle requires veterinary oversight and, in the United States, consultation with the Food Animal Residue Avoidance Databank (FARAD).

Milk residues are a special problem. After intramammary infusion of penicillin G in cows with clinical mastitis, milk antimicrobial screening tests agreed with liquid chromatography in 94% of samples, and with one exception all screening tests were negative by the end of the labeled milk withholding time [10]. That is reassuring for label-compliant use, but it does not cover extralabel use, and it does not cover every product. A single bulbar subconjunctival injection of 300,000 units of procaine penicillin G in dairy cows produced detectable beta-lactam activity in milk as early as 4 hours after treatment and as late as 22 hours after treatment, with 87% of cows testing positive at 10 hours [11]. That route is not on any label, and it shows how easily an unlabeled route can create a milk residue.

Species differences in depletion matter. In lactating buffaloes given procaine penicillin G and dihydrostreptomycin intramuscularly every 24 hours for 3 days, penicillin G concentrations fell below the maximum residue limit of 4 micrograms per kilogram by the 12th milking, which means the 9-milking withdrawal time set for bovine milk was not adequate for buffaloes [12]. In unweaned dairy calves fed milk replacer containing procaine penicillin G at 0.68 mg/kg once daily, penicillin G was not detected in muscle, but 13 of 24 carcasses had violative residues, 12 in liver only and 1 in liver and kidney, and a 21-hour withdrawal period was estimated [5]. Liver is the tissue most likely to hold a violative penicillin G residue in calves.

In swine fed a combination of sulfamethazine, chlortetracycline, and penicillin G at three times the recommended label dose for 14 days, penicillin G was detected in only one kidney sample at 0.018 mg/kg and one serum sample at 0.016 mg/L at zero withdrawal, and was not detected in any other sample [13]. That study does not establish a withdrawal time for penicillin G in swine, because the design and the combination drug make it unsuitable for that purpose. It does show that penicillin G depletes faster than sulfamethazine in that model.

The practical rule is simple. Use only a product labeled for the species and production class. Follow the label withdrawal time exactly. If extralabel use is necessary in a food animal, get veterinary direction and a residue avoidance recommendation before the drug is given, not after. Milk from treated cows must be discarded for the full labeled period, and milk from cows treated by any unlabeled route should be considered potentially contaminated until proven otherwise.

Clinical Relevance, Limitations and Common Mistakes

Penicillin G remains one of the most useful antibiotics in veterinary medicine because it is cheap, bactericidal, and effective against many of the gram-positive organisms that cause common infections in cattle, horses, and swine. Its limitations are equally important. It does not cover beta-lactamase producers, it does not cover Mycoplasma, and it does not cover most gram-negative enteric bacteria. Using it for a purulent infection without a culture is a guess, and in dairy mastitis that guess is wrong often enough that resistance surveys show high rates of penicillin G resistance in Staphylococcus aureus [2].

The most dangerous mistake is giving penicillin G to a rabbit, guinea pig, or hamster. The dysbiosis that follows can kill the animal within days, and it can happen after a single dose. Any owner who has been given a penicillin for one of these species should stop and call a veterinarian before the next dose.

The second most common mistake is using the wrong withdrawal time. Withdrawal times are product-specific and label-specific. They are not interchangeable between procaine and benzathine products, between intramuscular and intramammary routes, or between cattle and buffaloes. Extralabel use in food animals requires a residue avoidance plan, and PBPK modeling studies show that the correct interval after high-dose or repeated extralabel use can be substantially longer than the label [7][8][9].

The third mistake is assuming that oral penicillin G in calves is a treatment. Bioavailability is low, plasma concentrations are low, and the main measurable outcome is a residue risk in liver [5][4].

The fourth mistake is injecting procaine penicillin G intravenously or allowing it into a vessel. Procaine toxicity can cause excitement, tremors, and seizures.

The fifth mistake is failing to rotate injection sites. Repeated injection into the same muscle causes fibrosis, abscesses, and carcass trim loss.

Individual animals vary in how they absorb, distribute, and eliminate drugs, and a veterinarian who knows the animal, the organism, and the label is the right person to set the dose and the withdrawal time.

Frequently Asked Questions

What is Pen G used for in animals?

Pen G is used to treat infections caused by susceptible gram-positive bacteria, including streptococcal infections, Actinomyces pyogenes infections, foot rot, and some respiratory and soft tissue infections in cattle, horses, swine, dogs, and cats. It is also used as an intramammary infusion for mastitis in dairy cattle.

Can you give penicillin G to a rabbit or guinea pig?

No. Penicillin G can cause fatal dysbiosis in rabbits, guinea pigs, and hamsters by killing the gram-positive bacteria they need for hindgut fermentation. These species should never receive penicillin G or any other penicillin or cephalosporin unless a veterinarian has made a specific, informed decision and is monitoring closely.

How long does penicillin G last in the body?

Aqueous sodium or potassium penicillin G lasts only a few hours, procaine penicillin G lasts about 24 hours, and benzathine penicillin G lasts several days. The formulation, not the drug name alone, determines how often it must be given.

What is the difference between procaine and benzathine penicillin G?

Procaine penicillin G releases drug over about 24 hours and is usually given once daily. Benzathine penicillin G releases drug over several days and produces lower concentrations. Combination products contain both salts to get an early peak and a long tail.

Is penicillin G effective against E. coli or Pseudomonas?

No. Penicillin G does not cover E. coli, Pseudomonas, Klebsiella, or most other gram-negative bacteria, and it does not cover Mycoplasma species. A culture and susceptibility test is the reliable way to know whether penicillin G will work for a specific infection.

How long must milk be withheld after penicillin G in a dairy cow?

The milk withdrawal time is printed on the label of the exact product used, and it differs by formulation, route, and country. Follow the label. Do not use a withdrawal time from a different product or a different route, and do not use a bovine withdrawal time for buffaloes.

What happens if a food animal is slaughtered too soon after penicillin G?

Penicillin G residues can remain in liver, kidney, and muscle, and penicillin G is one of the antibiotics most likely to cause allergic reactions in people who eat contaminated meat. Violative residues can lead to condemnation of the carcass and, in some countries, legal penalties.

Can penicillin G be given by mouth to a calf?

It can be given by mouth, but absorption is poor. In one study, oral bioavailability in calves was 10.2% at 1 week of age and 7.4% at 5 weeks, and feeding milk containing penicillin G caused violative liver residues in most treated calves. Oral penicillin G in calves is a residue risk more than an effective treatment.

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