Procaine Penicillin G for Horses: Uses, Dosing & Side Effects

This information is for educational purposes only. Never give your pet any medication without your veterinarian's guidance. Dosing, frequency, and safety depend on your pet's specific health profile.

Procaine penicillin G is an injectable beta-lactam antibiotic used to treat certain bacterial infections in horses. It works by interfering with bacterial cell wall formation, so it is most useful against susceptible gram-positive bacteria and some anaerobic bacteria. In equine practice, your vet may choose it when culture results or the likely bacteria fit penicillin's spectrum.

The "procaine" part matters. Procaine is added to slow absorption after injection, which helps the medication last longer in the body than aqueous penicillin. That is why procaine penicillin G is typically given intramuscularly (IM) rather than intravenously. FDA labeling for veterinary products states for intramuscular injection only, and equine references list horse dosing as IM every 12 to 24 hours.

This medication is prescription-only and should be used under your vet's direction. Horses can have serious reactions if it is given incorrectly, if some medication accidentally enters a blood vessel, or if the horse is allergic to penicillin. Because of that, technique, site selection, and monitoring after each injection are a big part of safe use.

Your vet may prescribe procaine penicillin G for susceptible bacterial infections involving the skin, soft tissues, respiratory tract, reproductive tract, or deeper tissues. In horses, penicillin is commonly considered for infections caused by susceptible Streptococcus species and other penicillin-sensitive organisms. It may also be part of a broader treatment plan for wound infections, cellulitis, abscesses, and some post-procedure infections.

In equine medicine, procaine penicillin G is also used in selected cases of strangles-related complications, pleuropneumonia protocols, and pigeon fever/internal Corynebacterium pseudotuberculosis infections, depending on the horse's exam findings, imaging, and culture results. AAEP guidance for pigeon fever lists procaine penicillin G at 22,000 IU/kg IM every 12 hours, alone or combined with rifampin, for internal infection or ulcerative lymphangitis.

This is not a medication pet parents should start on their own. Not every swollen area, fever, nasal discharge, or wound needs penicillin, and not every infection will respond to it. Your vet may recommend culture and susceptibility testing, drainage, wound care, anti-inflammatory medication, or a different antibiotic depending on the location and severity of the infection.

In horses, published equine references list procaine penicillin G at 22,000 to 44,000 units/kg IM every 12 to 24 hours. The exact dose and schedule depend on the infection being treated, how severe it is, the product concentration, and whether your vet is combining it with other medications. AAEP guidance for pigeon fever specifically lists 22,000 IU/kg IM every 12 hours.

Many veterinary products contain 300,000 units/mL. That means the injection volume can become large in adult horses. For example, a 500 kg horse would receive about 36.7 mL at 22,000 units/kg, or about 73.3 mL at 44,000 units/kg, so your vet may divide the dose across multiple IM sites. Large-volume injections can increase soreness and swelling, which is one reason technique matters.

Do not change the dose, frequency, or route on your own. Procaine penicillin G should not be given IV. If high, sustained blood levels are needed quickly, references note that aqueous penicillin formulations may be used instead under veterinary supervision. If you are giving injections at home, ask your vet to show you exactly how to store, shake, draw up, place the needle, aspirate if instructed, and rotate injection sites.

Mild side effects can include stinging, soreness, or swelling at the injection site, plus digestive upset such as decreased appetite, diarrhea, or vomiting. Some horses also become temporarily uncomfortable after a large IM injection. Let your vet know if swelling is worsening, the horse seems painful to move, or the injection site becomes hot, firm, or draining.

More serious reactions need urgent veterinary attention. Horses can have allergic reactions to penicillin, including hives, facial swelling, fever, or trouble breathing. A separate and well-known equine risk is a procaine reaction, which can happen if procaine enters the bloodstream during or after injection. Horses may suddenly become excited, tremble, stagger, bolt, collapse, or act neurologically abnormal within minutes.

See your vet immediately if your horse has trouble breathing, severe weakness, collapse, seizures, marked agitation, or dramatic behavior changes after an injection. Even if the horse seems to recover, your vet still needs to know. Future antibiotic choices, injection technique, and monitoring plans may need to change.

Procaine penicillin G can interact with other medications, so your vet should review everything your horse receives, including prescription drugs, compounded medications, supplements, and recent injections. In general pharmacology, penicillins may have reduced effectiveness when combined with some bacteriostatic antibiotics that slow bacterial growth, because penicillin works best on actively dividing bacteria.

In equine practice, your vet may still intentionally combine penicillin with another antibiotic when the infection is complex or mixed. For example, AAEP guidance for pigeon fever lists procaine penicillin G used alone or with rifampin in some cases. That kind of combination should be chosen case by case, based on the suspected bacteria, culture results, tissue penetration, and antimicrobial stewardship.

Tell your vet if your horse has had a prior reaction to penicillin, cephalosporins, or injectable medications. Also mention kidney disease, severe diarrhea history, or any past neurologic-looking episode after an injection. Those details can change whether this drug is a reasonable option and how closely your horse should be monitored.