Major Uses

Penicillin is one of the earliest antibacterial drugs. Called penicillin G in its naturally occurring state, the medication effectively treats and prevents infections caused by many gram-positive and some gram-negative bacteria. Penicillin is also effective against anaerobic organisms, but it is susceptible to beta-lactamase hydrolysis.

Penicillin works by preventing production of the bacteria cell wall. Bacteria without a cell wall, such as Mycoplasma and Chlamydia spp., are not vulnerable to the effects of penicillin. This medication is most often used to treat skin infections, bladder infections, wounds, and tooth abscesses in cats, dogs, and other animals.

Common Precautions

Some animals develop loose stools or diarrhea after treatment with oral penicillin. This is because the antibiotic alters the bacterial population inside the intestinal tract. Cases of diarrhea should be reported to a veterinarian right away.

Chronic use of this medication may cause an overgrowth of non-susceptible organisms in the body. Overuse of broad-spectrum antibiotics can lead to development of super infections.

Systemic antibiotics should not be administered to animals suffering from shock, septicemia, or other serious illnesses. Absorption of the medication from the GI tract may be diminished or delayed. IV antibiotics are preferable in these cases.

Penicillin should not be given in conjunction with tetracyclines. Interactions are also possible with aminoglycosides, bacteriostatic antibiotics, and probenecid. Penicillin is known to cross react with other beta-lactam antibiotics, including cephalosporins, and should be used cautiously in animals with a known hypersensitivity to these drugs. Penicillin should not be used in animals with a known allergy or hypersensitivity to the drug. Overdose of penicillin does not usually cause adverse effects. However, any known or suspected overdose should be reported to a veterinarian right away.


Penicillin should be stored at room temperature, in its original packaging, out of the reach of children and animals. It should not be kept in a bathroom or over the kitchen sink. Penicillin injection should be stored between 36 and 46 degrees Fahrenheit.


Penicillin G is available in capsules and as an injectable. Other types of penicillin may be available in different forms.

The typical dose of penicillin in animals can range from 5,000 to 25,000 units/lb (10,000 to 50,000 units/kg) up to six times daily, depending on the type of infection present and the specific form of penicillin being used. If a dose of penicillin is missed, it should be given as soon as possible. If it is almost time for the next dose, the missed dose should be skipped and the normal schedule resumed. Two doses of this medication should not be given at the same time.

Doses may vary in different species, when the drug is given by a different route or concurrently with other medications, and with regards to a patient's age, breed, and health status. A veterinarian's dosing instructions and/or those printed on the medication label should be followed closely.

This information is for general reference only and should not be used for the diagnosis or treatment of any condition of your pet. It's intended as a general reference, this information may not include all possible uses, precautions, directions, reactions (including allergic), drug interactions, or withdrawal times. Always consult your local veterinarian and have your pet examined for any advice concerning the diagnosis and treatment of your pet, including which products and doses are most appropriate. Any trademarks are the property of their respective owners. VetDepot is not a pharmacy. All prescription products are dispensed by our Pharmacy Partner. Article last updated 2/2014.