PetMD Editorial
By PetMD Editorial on Nov. 7, 2012

Drug Info

  • Common Name: Penicillin G, Pen G, Procaine Penicillin
  • Drug Type: Antibiotic
  • Used For: Treatment and prevention of certain types of bacterial infections
  • Species: Dogs, Cats, Horses
  • Administered: Injectable, oral tablets, and liquid
  • How Dispensed: Some forms available by prescription only
  • FDA Approved: Yes


What Is Penicillin?

Naturally-occurring penicillin was the first antibiotic to be discovered. Many modified forms of penicillin are now available, but the natural form is still in use today.

Penicillin has a limited spectrum of activity, and many types of bacteria have developed resistance to this drug. For these reasons, veterinarians typically prescribe penicillin when they are reasonably sure that it will be effective against a particular infection or in combination with other drugs.

Penicillin is usually given by injection. Some types can be injected into a vein while others must be injected into a muscle or under the skin. An oral form is available but is rarely prescribed. It works best on an empty stomach but may be given with food if stomach upset occurs. 

How It Works

Penicillin kills susceptible bacteria by disrupting their cell walls and by preventing them from building a functional cell wall when they reproduce.

Storage Information

Store oral tablets and powders in a tightly sealed container at room temperature and away from bright light and moisture. Once the powder has been mixed with water, it should be refrigerated and disposed of after 14 days. Liquid penicillin for injection should be refrigerated. Some forms should be discarded after 24 hours if left at room temperature and after seven days if refrigerated. Follow the storage directions provided on the label.

What to Do If You Miss a Dose

Give the dose as soon as possible. If it is almost time for the next dose, skip the missed dose, and continue with the regular schedule. Do not give your pet two doses at once. Call your veterinarian if you have any questions.


Side Effects

Penicillin has a wide margin of safety. Its most serious side effects are associated with a serious type of allergic reaction called anaphylaxis. Gastrointestinal upset is also possible. Side effects that owners should watch for include:

  • Hives and other skin rashes
  • Itchiness
  • Facial swelling
  • Difficulty breathing
  • Vomiting
  • Diarrhea
  • Loss of appetite

Potential Drug Reactions

Do not mix injectable forms of penicillin in the same syringe with other medications unless directed to do so by a veterinarian. Penicillin should generally not be given with other antibiotics that inhibit bacterial growth because penicillin works by killing bacteria that are actively growing. 


Penicillin is generally considered safe for use in dogs, cats, horses, livestock, and many exotic pets. It can cause a disruption of the normal bacterial population within the gut of some species, especially rodents. Guinea pigs should not be given penicillin because they are prone to developing a potentially fatal condition that is caused by overgrowth of harmful bacteria within the intestinal tract and release of the toxins they produce.

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