Azithromycin (Zithromax®) for Cats and Dogs

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Overview of Azithromycin for Cats and Dogs

• Azithromycin, better known as Zithromax® or “Z-pack” in humans, is an antibacterial drug used in dogs and cats to treat a variety of conditions, including dermatological infections, respiratory tract infections, and urogenital infections.
• Azithromycin, invented in 1980 and derived from erythromycin, belongs to the azalide subclass of macrolide antibiotics. Like other macrolide antibiotics, it works by binding to the “P” site of the 50S ribosomal subunit of susceptible microorganisms interrupting RNA-dependent protein synthesis.
• Susceptible bacteria include Staphylococcus aureus, various Streptococci, some Hemophilus spp, certain Bacteroides spp, Borrelia burgdorferi, some Mycoplasma spp, and some species of Chlamydia.
• It is a reasonable antibiotic alternative for pets allergic to penicillin or in conditions where penicillin has failed to treat the condition.
• Following oral administration, azithromycin is rapidly absorbed and widely distributed throughout the body. Azithromycin’s bioavailability is 97% in dogs, 58% in cats, and its protein binding is 7 to 51%, depending on the plasma concentration.
• Rapid distribution of azithromycin into tissues results in significantly higher azithromycin levels than in plasma. The half-life of azithromycin is long, lasting up to 90 hours in dogs. In cats, the tissue half-life is 13 to 72 hours, depending on the tissue. Azithromycin undergoes some hepatic metabolism, but the majority of an administered dose is excreted unchanged in bile.
• Azithromycin is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian. It is not a controlled substance.
• Azithromycin is registered for use in humans only. This drug is not approved for use in animals by the Food and Drug Administration (FDA), but it is prescribed legally by veterinarians as an extra-label drug.

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Brand Names and Other Names for Azithromycin

• Human formulations: Multiple human preparations include Zithromax® (Pfizer), Zithro, Zaraxin, Clamelle, Zedbac, Zmax, AzaSite, as well as various generic formulations.
• Veterinary formulations: None

Uses of Azithromycin for Dogs and Cats

The most common uses of azithromycin in pets are for the treatment of infections caused by susceptible microorganisms involving the skin, respiratory tract, and urogenital systems. Additional uses are for the treatment of otitis media (inner ear infections), bacterial causes of keratitis and conjunctivitis, cyclosporine-induced gingival hyperplasia, acute cytauxzoonosis, toxoplasmosis, and Babesia gibsoni infections. There is also evidence that azithromycin may aid in the treatment of viral papillomatosis in dogs.

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Key Points About Giving Azithromycin to Your Pet

• Azithromycin is an antibiotic used in dogs and cats to treat respiratory, urinary, and skin infections.
• Azithromycin is not available over the counter. It is a prescription drug and should only be administered under the guidance of a licensed veterinarian. A prescription can be obtained from your veterinarian and filled at veterinary hospitals or most human pharmacies.
• The dosage, duration, and frequency of treatment varies depending upon the underlying condition, response to the medication, and the development of adverse effects.
• Azithromycin can interact with other medications.
• While generally safe and effective, azithromycin does have side effects in some pets with diarrhea being the most common.

Precautions and Side Effects

• While azithromycin is generally safe and effective when prescribed by a veterinarian, it can cause side effects in some animals. Most importantly, azithromycin should not be used in animals with known hypersensitivity or an allergy to azithromycin or other macrolide antibiotics. Note: Other macrolides include clarithromycin, erythromycin, and roxithromycin. If your pet has had a reaction to any of these medications, inform your veterinarian immediately.
• It is important to understand the general principles of antibiotic usage and that frequent or prolonged administration can lead to resistant infections. Your veterinarian will help guide you to what is appropriate for your pet.
• Although azithromycin is commonly used in dogs and cats, it was created for and is licensed for use in humans only. Therefore, most available research focuses on human use and side effects. There is substantial documentation of liver problems including hepatotoxicity and cholestatic jaundice in humans. Additional side effects in humans include hearing abnormalities, taste or smell loss, angioedema, severe diarrhea caused by Clostridium difficile, and aggravation of myasthenia gravis symptoms.
• In dogs and cats, azithromycin should be used with great caution (if at all) when there is a pre-existing hepatic (liver) or renal (kidney) disease and avoided with the failure of these organs. The most common side effects relate to the gastrointestinal tract, such as vomiting, diarrhea, or abdominal pain. Cardiac arrhythmias, including ventricular tachycardia, may be precipitated by azithromycin. Renal dysfunction, including interstitial nephritis and acute renal failure, may occur secondary to azithromycin treatment and liver function may be affected. Eye irritation may occur in pets receiving the ophthalmic formulation. Ergot toxicity may occur when ergotamine or dihydroergotamine are concurrently administered with azithromycin.
• Note: Call your vet for recommendations if you believe your pet is suffering from an adverse reaction.

Drug Interactions

Azithromycin can interact with certain medications that include:

• Azithromycin may elevate serum digoxin levels in cardiac patients taking digoxin. It can also increase serum levels of theophylline and methylprednisolone.
• Azithromycin can cause a decrease in the clearance of triazolam and midazolam, increasing pharmacologic effects.
• Pimozide is contraindicated in patients receiving azithromycin, and vice versa (death may result).
• Animals being treated with cisapride should not be given azithromycin or other macrolide antibiotics.
• Drugs metabolized by cytochrome P450 (e.g., carbamazepine, terfenadine, cyclosporine, hexobarbital, and phenytoin) will have their serum levels elevated by azithromycin.
• Oral antacids reduce the absorption of azithromycin. When used, drugs should be separated from each other by 2 to 3 hours.
• Azithromycin is incompatible with chloramphenicol when given via subcutaneous route.
• Phenobarbital, a common seizure medication, can alter metabolic enzymes, resulting in ineffective azithromycin therapy.
• Other drugs that should be used with caution include sotalol, ondansetron, dolesetron, ketoconazole, itraconazole, and enrofloxacin.
• If your pet is taking any of the medications listed above, discuss the use of azithromycin with your veterinarian. They can help you understand the risk relative to the benefit and if this is the right medication for your pet.

How Azithromycin Is Supplied and Stored

Most veterinary and human pharmacies inventory tablet sizes (250 and 500 mg) and powder for oral suspension (200 mg/5 mL). Other formulations are generally available by special order.

Azithromycin is supplied as:

• Tablets: 250 mg, 500 mg, and 600 mg film-coated oral tablets. Can be stored at room temperature.
• Capsules: 250 mg. Can be stored at room temperature.
• Powder for injection: 500 mg (lyophilized) in 10 mL vials. Stable for 7 days when reconstituted and stored under refrigeration and for 24 hours when stored at or below 86 degrees Fahrenheit.
• Powder for oral suspension: 100 mg/5 mL, 200 mg/5mL, & 1 g/packet. Store between 41- and 86-degrees Fahrenheit.
• Ophthalmic solution: 1% in 2.5 mL bottle. Should be refrigerated and discarded 14 days after opening. For topical use only.
• Toothpaste: Azithromycin toothpastes may be made at compounding pharmacies in strengths that range from 5% to 8.5% in a variety of flavors. For oral use only.

Azithromycin Dosage for Cats and Dogs

• Azithromycin should never be administered without first consulting your veterinarian. It is a prescription product and cannot be purchased over the counter. It may be given with or without food. For pets that suffer from nausea, vomiting, or diarrhea, it is better to give medication with food. If you miss a dose of azithromycin, give the next dose as scheduled. Do not double the dose.
• The dosage recommended for your pet will vary depending upon the underlying condition being treated. For example, it may be dosed at 15 mg/kg twice daily for 7 days in dogs with a coccidiosis infection or dosed at 5 mg/kg once daily for dogs with a Giardia infection.
• While there is a substantial variation in dosages, the usual dosage in dogs is 2.5 to 5 mg per pound (5 to 10 mg/kg) orally once daily for up to 7 days. In cats, the usual dosage is 2.5 to 7.5 mg per pound (5 to 15 mg/kg) orally every 12 to 24 hours for up to 7 days. Some veterinarians may recommend gradually decreasing the frequency to every 3 to 5 days for a series of doses when treating some conditions (such as respiratory infections in cats).
• Again, the dosage and duration of administration depends on the condition being treated, response to the medication, and the development of adverse effects. Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your dog or cat feels better, the entire treatment plan should be completed to prevent relapse.