Azithromycin for Cats: Dosage, Safety & Side Effects

Azithromycin is an antibiotic commonly used to treat certain bacterial infections in cats. In this article, you’ll learn more about what azithromycin does, conditions it may be indicated for, side effects to look out for, and some frequently asked questions.

Azithromycin For Cats Overview

Medication Type:

Macrolide antibiotic

Form:

Oral tablet, oral suspension

Prescription Required?:

Yes

FDA Approved?:

No

Brand Names:

Zithromax

Common Names:

Azithromycin

Available Dosages:

Note: The tablet doses available for azithromycin are typically too large for cats: 250mg, 500mg, 600mg. The liquid suspension is most often used, available in 20mg/ml, 40mg/ml concentrations. Bottle sizes include 15ml and 30ml sizes.

About Azithromycin for Cats

Azithromycin, the common brand name for which is Zithromax, is classified as a macrolide antibiotic. It does have a fairly broad spectrum of action against many different kinds of bacteria.

Azithromycin is most often used in cats for upper respiratory infections, especially conditions that are chronic, recurrent, or refractory to other antibiotic therapies.

Azithromycin is listed as a Critically Important, Highest Priority antimicrobial for human medicine, making its indication for long term use in cats with chronic rhinosinusitis (chronic inflammation/infection of the nasal passages and sinuses) controversial, as long term use could cater to antimicrobial resistance to the drug. It is therefore may be used more in select cases and not as a first-line choice.

It reportedly has a quick effect against infections caused by Bartonella (the bacteria that causes “cat-scratch disease” in people) although clinically concerning infections in cats caused by Bartonella tend to be uncommon.

It is also thought to have some activity against certain protozoal microorganisms like Toxoplasma and Babesia as well, though better efficacy may be seen in combination with other appropriate medications.

Azithromycin is a bacteriostatic antibiotic, but this does not mean it is “weaker” than a bactericidal antibiotic like Clavamox. Bacteriostatic antibiotics do kill susceptible bacteria, they simply require a higher concentration to do so.

A 2018 study in Clinical Infectious Diseases comparing bacteriostatic and bactericidal antibiotics in people highlights this distinction. Researchers found that not only were bacteriostatic and bactericidal antibiotics about equivalent in efficacy most of the time, but that in cases where there was a difference, bacteriostatic antibiotics often appeared superior and more cost effective.

Unlike most antibiotics, azithromycin also has some anti-inflammatory and immune-modulating effects. Although these effects are not well understood in animals, these benefits likely contribute to improvements seen in cats with chronic rhinosinusitis and stomatitis (inflammation in the mouth).

Also while not very well understood in animals, azithromycin does have some promotility effects on the gastrointestinal tract in people (meaning it helps the GI tract move food along) and some of these effects have also been reported in dogs and cats.

What Does Azithromycin Do for Cats?

A unique characteristic of azithromycin is that it can remain in tissues much longer than other antibiotics.

Azithromycin is an effective antibiotic against most of the bacteria typically susceptible to it. Veterinarians commonly determine antibiotic use based on bacterial culture whenever possible or at least based on the likelihood of infectious organisms present, and the body system affected.

As with most antibiotics, azithromycin has good levels of efficacy against certain bacteria, while others may be resistant or just not susceptible to it.

For example, an upper respiratory infection in a cat caused by the bacteria Bordetella, or uveitis (inflammation in the eye) caused by Bartonella, will typically be susceptible, but it has no effect against Chlamydophila felis that causes conjunctivitis in cats, or anemia caused by Mycoplasma haemofelis in cats.

Azithromycin also is not effective against viral causes of upper respiratory infections in cats, like feline herpesvirus and calicivirus, which are actually by far the most common underlying causes in kitties.

A unique characteristic of azithromycin is that it can remain in tissues much longer than other antibiotics. This unique absorption of azithromycin allows for less frequent dosing. This makes it a good and more cost-effective choice for certain susceptible infections that may require weeks of treatment.

Because of these properties, a veterinarian may consider azithromycin for upper respiratory infections, especially those involving the nasal passages and sinuses, that are very chronic or recurrent in nature and may require extended periods of treatment to resolve.

Anecdotally, azithromycin has also been useful in cats with stomatitis, which is a chronic inflammatory condition of the tissues in the mouth. This is thought to be related to its anti-inflammatory and immune system modulating effects.

While cost-effective, it may be considered controversial for this purpose, since extended or life-long use of it can also lead to antimicrobial resistant strains of bacteria. It should therefore only be used for such purposes in select cases under advisement by a veterinarian.

Side Effects of Azithromycin for Cats

Azithromycin is generally a safe and well-tolerated drug in cats. Like many antibiotics, it has the potential to cause digestive upset, including vomiting, reduced appetite, and diarrhea. Overdoses of azithromycin may be more likely to lead to these adverse effects and its pro-kinetic effects on GI motility could also cause abdominal discomfort from cramping.

Azithromycin is primarily metabolized by the liver, so it should be used cautiously in any cats with underlying liver disease or dysfunction.

Like any drug, azithromycin can have drug interactions with some other medications, most notably -azole antifungals like itraconazole, that could lead to an alteration in heart rhythm. Your veterinarian should always be consulted before starting any new medication, so that any potential interactions can be reviewed.

A last important note is that azithromycin can sometimes be confused with azathioprine, which is a very different drug used as an immunosuppressive and is not commonly used in cats.

Miscommunication could occur over the phone with a pharmacy, through mis-spellings, or misinterpretations of handwritten scripts. Azithromycin typically is called into a human pharmacy, so always make sure to double check your kitty’s prescription when you pick it up.

Azithromycin for Cats Dosage

In cats, it is most common for the liquid oral suspension to be prescribed, as the available tablet sizes are too big for dosing in most cats.

The dosage for azithromycin can differ depending on the type of infection being treated. Treatment frequency and length can also vary quite a bit. For this reason, the actual dosage for a kitty should only be provided by an attending veterinarian to ensure the drug is being used properly and responsibly.

For general understanding of its use though, it is important to include its unique dosing characteristics, which differ from other antibiotics.

Because azithromycin can reach higher and longer concentrations in tissues, there are protocols that allow for it to be given once a day for several days, and then only every other day or every third day, especially if treatment is to be continued for several weeks or long term. Other protocols may include higher doses used only a couple of times a week.

These protocols are used most often for chronic upper respiratory infections and stomatitis but are not appropriate for all indications. That’s why it’s important to adhere to a veterinarian’s instructions for proper prescription dosing of this drug.

In cats, it is most common for the liquid oral suspension to be prescribed, as the available tablet sizes are too big for dosing in most cats. Because azithromycin is a human medication and is used off-label in pets, the liquid flavoring is catered to people and some cats may not like the taste. A veterinarian may recommend getting a compounded form of it to help improve palatability and effective dosing.

Conclusion

Azithromycin can be a useful antibiotic in cats, especially where long-term use is needed or where cost is a significant concern. It has some anti-inflammatory and immune-modulating properties as well as some unique dosing characteristics that set it apart from other antibiotics.

However, it cannot be used for all types of infections and like all antibiotics, should be used judiciously under prescription and advisement of a veterinarian.