Azithromycin is an antibiotic that is sometimes prescribed to cats for bacterial infections, especially in the respiratory tract. It is always important to follow your veterinarian’s instructions precisely when giving this drug, like all medications, to avoid complications. This drug can have side effects and adverse reactions in cats, although they are not very common.

What Is Azithromycin?

Azithromycin is an antibiotic that falls into the class of drugs called macrolide antibiotics. These are drugs primarily used to treat specific bacterial infections. This drug works by preventing the bacteria from making critical proteins. In most cases, azithromycin has bacteriostatic effects, meaning that it does not kill the bacteria outright, but slows their growth down enough for the cat's immune system to do the rest. 

Azithromycin Uses in Cats

In cats, azithromycin is most commonly used to treat certain kinds of bacterial infections. These are often bacteria that cause upper respiratory infections, such as Bordetella, Streptococcus, Mycoplasma and Staphylococcus species. It is not considered the first choice antibiotic for these infections most of the time, however, it does have some properties that make it a good choice in the right circumstances.

Your veterinarian will decide if antibiotics are warranted and may wish to perform a culture and sensitivity before reaching for an antibiotic like azithromycin. In many cases of upper respiratory infections, they may originate as an uncomplicated viral infection, which may not require antibiotic use every time sneezes and sniffles are heard.

Dosage and Administration

Azithromycin comes as a capsule, tablet, or a liquid formulation that is given by mouth. It also comes in an injectable form that is given under the skin as a subcutaneous injection and would likely be given in your vet’s office. You should always follow your veterinarian’s instructions carefully for giving azithromycin as this medication is dosed by weight, so your cat’s dose will be specific to its size.  

At this time, there is no veterinary-specific formulation so a human formulation is prescribed to cats. The most common brand name is Zithromax. In some cases, it may also be possible to have this medication compounded into a form that your cat will find more tasty. A compounding pharmacy can often make a liquid or chewable formulation that is flavored like tuna, chicken, or another one of your cat’s favorite treats.  

Side Effects

In most cases, this medication is safe for use in cats and has limited side effects. Like many antibiotics, however, it can sometimes upset their digestive tract. This can lead to signs like vomiting, diarrhea, belly pain, and/or decreased appetite if they feel nauseous. For these reasons, it is usually recommended to give this medication on a full stomach, or shortly after eating, to reduce the likelihood of tummy troubles.  

This medication is removed from circulation by the liver, like many drugs. In unusual cases, it can cause elevations in liver enzymes that are measured on routine blood tests.

Finally, the most unusual side effect of azithromycin would be hypersensitivity reactions. These are the kinds of reactions that cannot be predicted, but in a very small number of patients can cause signs of severe allergic reaction such as hives, swelling of the face, or difficulty breathing. Although rare, this can happen with almost any medication, so you need to have your pet seen by a veterinarian immediately. In all cases of prescribed medicine, your veterinarian has weighed the benefits and risks, and prescribed a medication when the benefits outweigh the risk to your pet.

In the event that your cat shows any of these signs, or any other abnormal behaviors or physical signs, it is always best to consult your vet right away before continuing the medication.  

Risk Factors

It is always best to speak with your veterinarian to review any of your cat's ongoing medical conditions and/or current medications your cat is taking before starting a new medication. There may be some risk factors related to these situations, including:

• Liver disease: Cats that have existing liver disease may be more at risk for complications if using this medication at its normal dose.
• Drug interactions: Some medications could interfere with the effectiveness of azithromycin. Always notify veterinarians of all the medications your cat is taking at any given time. 
• Antibiotic resistance: Antibiotic resistance means that over time, the population of bacteria in your cat’s body can mutate and evolve resistance to an antibiotic. If that happens, it means your cat will stop responding to the medication and its signs of illness may come back and not respond to treatment.

Be on the lookout for the initial signs of your cat’s illness, and be sure to notify your vet if you notice those signs returning despite continued treatment.