Benzathine Penicillin, Ambi-Pen, Benza-Penc, Crystiben, Durapen, Procaine Penicillin, Aquacillin, Crysticillin 300 A.S, Pen-Aqueous, Penicillin G Procaine, Pfi-Pen G, Sterile Procaine Penicillin G, Potassium Penicillin.

Other Penicillin Combination Products:
Azimycin, Bicillin, Combiotic, Diathal, Duo Biotic, Pen-Dihydro, Strepta-Cillin, Stress-Biotic.

Penicillin is a naturally occurring antibiotic with activity primarily against certain classes of gram-positive bacteria.

Penicillin is indicated in the treatment of infections caused by organisms sensitive to this drug. These include “strangles” and a wide variety of skin, soft tissue, and urinary tract infections caused by many species of Streptococcus and Staphylococcus (“Strep” and “Staph”) species, as well as other bacteria.

With any infection, appropriate cultures should be taken and the sensitivity of the bacteria to penicillin established prior to beginning therapy.

Penicillin is available as an intravenous preparation (potassium penicillin); as a long-acting intramuscular form (procaine penicillin); and as an extended duration (approximately 48 hours) preparation (benzathine pencillin).

For the intramuscular preparations, the manufacturer’s recommended dose is 3000 units/lb once daily. Higher dosages or multiple injections per day may be used at the veterinarian’s discretion, depending upon the severity of the infection, and/or the nature of the organism involved. Therapy may also be initiated with, or supplemented by, intravenous treatment at the veterinarian’s discretion.

Penicillin should not be used in any horse known to have an allergy to it or to related antibiotics, such as ampicillin or oxacillin. Cross-allergies to cephalosporins may also exist. Severe allergic reactions to the penicillin family of antibiotics have been reported including skin changes and respiratory distress collapse and even death.

Evidence suggests that people with asthma or multiple allergies are more likely to have an allergy to penicillin. This connection has not been established in the horse, but it would be prudent to bear this in mind when selecting an antibiotic for use in a horse with multiple allergies or heaves

Sensitivity to the procaine contained in procaine penicillin, or combination products containing procaine penicillin, is also possible. In humans sensitivity to procaine is usually manifested as mental changes such as confusion, agitation anxiety, weakness depression or disorientation. The distinction between reactions to penicillin and reactions to procaine has not been made in horses. In most cases therefore the drug will be discontinued anyway should any adverse reactions appear If penicillin is the drug of choice, and the condition is serious the veterinarian may be able to make a distinction between procaine and penicillin sensitivity by injecting a small amount of procaine under the skin, and watching for the development of redness swelling, or a bump. Another clue that procaine, and riot penicillin was causing the reaction would be if the horse’s condition improved following the administration of antihistamines.

When using intramuscular preparations injections must go deeply into a large muscle, but be made well away from any nerve or artery. These preparations can be irritating and when administered improperly can cause nerve damage. Anyone administering this drug should be thoroughly familiar with safe injection sites. No more than 10 cc should be given per injection site for an adult horse, 5 cc for a foal, and even less in very small foals.

Intravenous injection of penicillin should only be done by the veterinarian. This is because any allergic reaction that develops is likely to be rapid and more severe requiring immediate treatment. Overdose has been reported to cause convulsions in humans.

Penicillin should be stored in the refrigerator.

Allergic/hypersensitivity reactions, such as hives or respiratory distress, can occur in some horses. Immediate cessation of therapy should occur, as well as immediate veterinary attention if the veterinarian is not on the premises.


A wide range of side effects has been reported in humans taking penicillin including rashes, diarrhea, nausea, and other problems. Any horse that develops unexplained problems shortly after beginning penicillin therapy should have the medication stopped, and be evaluated by a veterinarian.

Penicillin should not be given orally to horses. It has the ability to seriously disrupt the normal population of microorganisms in the intestinal tract, resulting in diarrhea and abdominal discomfort.

When penicillin (or any antibiotic) is used for prolonged periods, there is the possibility of the horse developing a superimposed fungal infection or superinfection, with resistant bacteria. Any new symptoms suggestive of this fever, malaise, foul odor to urine or wound, inflammation of wound) must be investigated by the veterinarian immediately.

Penicillin should not be used in conjunction with the bacteriostatic class of antibiotics (e.g., Bactrim or tetracycline; as these two classes of antibiotics work against each other.

Studies in laboratory animals have demonstrated the safety of penicillin during pregnancy. There are no studies available specifically for the horse.

Penicillin is excreted in the milk in other species and probably also in the horse. This is not likely to be of concern unless it interferes with the treatment of the foal for another condition. Caution should be used, and the foal carefully monitored, for signs of side effects or fungal infections.

The combination penicillin products are of three types:

Benzathine and procaine penicillin

Procaine penicillin and another antibiotic (streptomycin; Procaine penicillin, streptomycin, and a corticosteroid

Although these products might seem convenient, equine internal medicine experts raise some important questions on their use. The combination benzathine and procaine penicillins were designed to decrease the number of times the horse needs to be injected. Blood levels of penicillin are not as high or as well maintained as when procaine penicillin is used alone. This could lead to inadequate treatment or the development of resistant strains of bacteria. On the other hand, a special program for difficult-to-treat horses could utilize these combinations effectively by designing an individualized program, perhaps combining an initial intravenous dose of penicillin, treatment for a few days with procaine penicillin, and finishing the program with the benzathine/procaine combination.

Combinations of penicillin and streptomycin were designed to increase the number of organisms that would be susceptible. However, for best activity streptomycin must be given three times daily. Also, with some of the combinations there is an inadequate total daily dose of streptomycin. These drugs could be used if streptomycin is present in adequate amounts and the combination product given twice daily, with a supplemental dose of streptomycin only’, given between the two doses of the combination product.

Finally combination drugs containing premixed amounts of penicillin, streptomycin, and steroids should be used only in rare circumstances, if at all. Corticosteroids decrease the body’s ability to fight infection and are directly contraindicated when infection is present. There are rare occasions when the degree of inflammation involved with an infection is causing significant problems of its own. In those cases, the veterinarian may decide that a short course or single injection of a corticosteroid is warranted, if the infection seems to be responding well to the antibiotics given. However, this is definitely a decision that can only be made by the veterinarian and the use of antibiotic/corticosteroid combination products should not be routinely done.

Detection Time Information: Within the last five years there have, to our knowledge, been no medication violations called for penicillin detected in horse urine. There has, however, been at least one disciplinary action for the treatment of a horse on race day with a penicillin preparation and another event where an injectable preparation of penicillin was confiscated and the ruling noted as a penicillin ruling. As far as penicillin is concerned, it has not been called a medication violation as such from horse urine, but you can get into trouble if you are discovered with a syringe with penicillin in it in the wrong place or at the wrong time.

THE PROCAINE IN PROCAINE PENICILLIN IS ONE OF THE MOST COMMONLY CALLED VIOLATIONS AND PROCAINE HAS BEEN REPORTED IN THE SClENTIFIC LITERATURE TO BE DETECTED IN HORSE URINE FOR 18 DAYS AFTER THE LAST DOSE OF THIS DRUG. Since these are laboratory experiments with small numbers of horses, the likelihood of the drug being detectable for significantly longer periods Up to 30 days or longer) in individual horses must be borne in mind.

Benzathine penicillin is an extremely rare source of medication violations.

Rules of Governing Bodies

Standardbred and Thoroughbred Racing: Standardbred and thoroughbred racing operates under a basic drug-free policy. However, individual state racing commissions may permit individual therapeutic drugs, often at certain previously established “permissible” blood levels. Check with the commission in your state.

AQHA: Sanctioned Quarter Horse shows operate under a drug-free policy. For racing, individual state racing commissions may or may not allow certain levels of individual drugs. Check with the commission in your state.

AHSA: This is a forbidden substance for horses showing under No Foreign Substance rules. This is an unrestricted therapeutic substance for horses showing under Therapeutic Substance rules. HOWEVER, ALL PENICILLIN PREPARATIONS, EXCEPT FOR INTRAVENOUS POTASSIUM FORM, CONTAIN THE FORBIDDEN SUBSTANCE PROCAINE, AND WILL CAUSE A POSITIVE FOR 14 DAYS AFTER LAST ADMINISTRATION.

Horseman’s Comments

I always liked to have a bottle of penicillin on hand in the barn. Not that I was using it for every little thing, as I’ve seen some other horse folks do, but in case of puncture wounds or when hoof abscess threaten the spread of infection up the leg etc. In all these years I have not encountered one horse that has had an allergic reaction to the penicillin, out of more than 200 horses treated. Like I’ve said, very practical medicine to have on hand. It is inexpensive and unopened, it keeps for a long time. (Check the expiration date before purchase, available in many feed stores and mills.