Drugs and medications image
Drugs and medications image EquiMed


pen-ah-SIL-en - Pronunciation guide

Brand Names

  • Agri-Cillin
  • Ambi-Pen
  • Amp-Equine
  • Penicillin Injectable


Penicillin is an antibiotic produced by a form of mold and used medically as a bactericidal to kill bacteria that causes diseases.  Penicillin, in various forms, is one of the most commonly used antibiotics in veterinary medicine.

Different groups of penicillin are effective against different types of bacteria. Bacteria are often divided into two groups that have been catalogued because of their reaction to staining, a process developed by a Danish physician, Hans Christian Gram, that reveals differences in biochemical and structural properties. Simply put, when stained according to research custom, Gram-positive bacteria remain purple because they have a thick cell wall, and Gram-negative bacteria lose color because they have a thin cell wall.

Natural penicillins work against Streptococcus and other Gram-positive bacteria, such as Bacillus, Listeria, Stapylococcus, Enterococcus, and Clostridium.

Gram-negative bacteria, or proteobacteria, include Escherichia coli and Salmonella, among others.

Aminopenicillins, such as ampicillin, are effective against a broader spectrum of bacteria, including both Gram-positives and Gram-negatives.  Extended spectrum penicillins, including ticarcillin, are active against a broad range of bacteria, including Pseudomonas, which are often resistant to other antibiotics.

Penicillins are easily and widely distributed throughout most organs and tissues of the body shortly after intra-muscular or intravenous administration.  They are poorly absorbed when administered orally.


Penicillin in its various forms is used to treat a wide range of infections caused by numerous forms of bacteria.  Given the number of forms of penicillin available and the variety of uses, consultation with a knowledgeable veterinarian is important at the earliest sign of infection or disease.

Dosage and Administration

Method Dosage Concentration Period Duration
Penicillin G, procaine
Intramuscular injection1 6600-16000 IU/kg 300000 IU/ml Daily Up to 7 days
Penicillin G, sodium or potassium
Intravenous or Intramuscular1 injection 20000 IU/kg 5000000 IU/ml Every 6 to 8 hours Up to 7 days


  • 1The suspension should be administered by deep intramuscular injection within the fleshy muscles of the hip, rump, round or thigh, or into the neck, changing the site for each injection. Do not inject subcutaneously, into a blood vessel, or near a major nerve.

    Use a 16 or 18 gauge needle, 1.5 inches long. The needle and syringe should be washed thoroughly before use. The needle and syringe should then be sterilized by placing in boiling water for 15 to 20 minutes.

    The injection site should be washed with soap and water and painted with a germicide such as tincture of iodine or 70% alcohol. The product should then be administered by using the following procedure:

    Daily treatment should be continued for at least 48 hours after temperature has returned to normal and all other signs of infection have subsided. Animals treated with Penicillin Injectable Suspension should show noticeable improvement within 36 to 48 hours.

    1. Warm the vial to room temperature and shake thoroughly to ensure uniform suspension.
    2. Wipe the rubber stopper on top of the vial with a piece of absorbent cotton soaked in 70% alcohol.
    3. Inject air into the vial for easier withdrawal.
    4. After filling the syringe, make sure that the needle is empty by pulling back the plunger of the syringe until a small air bubble appears. Then detach the needle from the syringe.
    5. Insert the needle deep into the muscle, attach the syringe and withdraw the plunger slightly. If blood appears, withdraw the needle and insert it into a different location.
    6. Inject the dose slowly. Do not massage the site of injection.
    7. Not more than 10 mL should be injected in one location.
  • Extra-label use of drugs in treating animals is allowable only by licensed veterinarians within the context of a valid veterinarian-client-patient relationship, and does not include drug use in treating animals by the layman (except under the supervision of a licensed veterinarian).
  • The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your equine appears to feel better, the entire treatment plan should be completed to prevent relapse.
  • This medication may be available in forms and concentrations not noted in the above table. Always check the label and literature provided with the medication about the form and concentration and DO NOT USE the calculator if the information differs.

Many formulations are available. Consultation with a veterinarian is very important as to dosage and administration.

Side Effects

Side effects with penicillin usage are rare in animals.


Penicillins should not be given to animals that have had an allergic reaction to any antibiotics in this group or to cephalosporin antibiotics because of the possibility of cross-reactivity.  In all cases, the advice and prescription by a knowledgeable veterinarian is of great importance in determining what is best for each individual animal's case.

Several forms of penicillin are FDA approved for use with horses.  Penicillins are prescription drugs and U.S. federal law restricts these drugs to use by or on the lawful written or oral order of a licensed veterinarian.


Penicillin antibiotics should not be combined with bacteriostatic antibiotics, such as erythromycin, tetracycline, or neomycin.

Ampicillin may affect the activity of rifampin.

Large doses of certain penicillins have been associated with bleeding in humans.  These penicillins should be closely monitored when used in horses, especially those receiving oral anticoagulants or heparin.


Very high doses or overdoses of penicillin may cause neurological signs.  Horses with decreased kidney funtion may be more likely to experience adverse effects.


Agri-Cillin Penicillin InjectionAgri-Cillin Penicillin Injection

Pen-Aqueous Penicillin InjectionPen-Aqueous Penicillin Injection



About the Author

EquiMed Staff

EquiMed staff writers team up to provide articles that require periodic updates based on evolving methods of equine healthcare. Compendia articles, core healthcare topics and more are written and updated as a group effort. Our review process includes an important veterinarian review, helping to assure the content is consistent with the latest understanding from a medical professional.