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Azithromycin (Zithromax)
Revised: March 23, 2024
Published: January 09, 2009

(For veterinary information only)

The size of the tablet/medication is NOT an indication of a proper dose. Never administer any drug without your veterinarian's input. Serious side effects or death can occur if you use drugs on your pet without your veterinarian's advice. 

It is our policy not to give dosing information over the internet.

Brand Name: Zithromax

Available as 250 mg, 500 mg, 600 mg tablets, and oral suspension (liquid)


Azithromycin is a semi-synthetic antibiotic of the macrolide class, the same class as erythromycin and tylosin. This class of antibiotics acts to inhibit bacterial protein synthesis by inhibiting a cellular structure called a 50S ribosome, a structure that only certain bacteria have and use to make internal proteins. Because non-bacterial cells, like mammal cells, use a different type of ribosome (called a 60S ribosome), this difference in the ribosome structures can thus be exploited. In other words, macrolide antibiotics will interfere with the 50S ribosome but will have no effect on the 60S ribosome, meaning disruption in protein manufacture for bacteria with no effect on mammal cells.

How this Medication is Used

Azithromycin has activity against many bacterial species, including Staphylococcus, Streptococcus, and Lyme disease.

Azithromycin and atovaquone (a quinine derivative) combine to form an effective treatment against Babesia gibsoni, a blood cell parasite.

Azithromycin has been able to combat the overgrowth of gum tissue (gingival hyperplasia), which is sometimes a side effect of cyclosporine, an immunomodulator. To facilitate this use, an azithromycin toothpaste has been produced.

Azithromycin is also used to reduce the number of viral papillomas (viral warts) in dogs, though this use is somewhat controversial as to how well it works. 

Azithromycin is often used for cats with chronic nasal disease/upper respiratory infection after other antibiotics have been found ineffective. 

Azithromycin can also improve gastrointestinal mobility, though it is usually used for its antibiotic properties.

Azithromycin is often compared to its cousin erythromycin, and found to have a longer half-life in dogs and cats as well as better absorption of oral dosages. This means it lasts longer and gets into the body easier.  

This medication may be given with or without food.

If a dose is skipped, do not double up on the next dose but simply resume dosing as usual with the next scheduled dose.

Store away from light and store at room temperature. Do not refrigerate. 

If you miss a dose, do not double up on the next dose. Simply give the medication when it is remembered or pick up with the next dose, allowing at least the proper interval between doses according to the label instructions.

Side Effects

At higher doses, vomiting has been reported, though generally, this is felt to be less of a problem than with erythromycin. Azithromycin overdose manifests as diarrhea, vomiting, and abdominal cramps.

Interactions with Other Drugs

Macrolide antibiotics interact with cisapride and should not be used together. This is because of a heart rhythm disturbance that results with concurrent use. Other drugs that can have similar reactions with azithromycin include sotalol (a heart medicine), the "azole" antifungal medications (ketoconazole, itraconazole, etc.), ondansetron and dolesetron (nausea medications), and quinolone class antibiotics (enrofloxacin, marbofloxacin, etc.). 

When theophylline is used concurrently with azithromycin, there is an increase in the potential for toxic effects of theophylline.

Cyclosporine blood levels can increase when azithromycin is used concurrently.

Oral antacids interfere with intestinal absorption of azithromycin after oral dosing. Ideally antacid administration and azithromycin should be separated by two hours.

Concerns and Cautions

Because azithromycin is largely removed from the body via the liver (in bile), normal liver function is needed to remove the drug from the body. If liver function is impaired, dosing adjustments may be required.

The oral suspension should not be refrigerated and spoils after 10 days (and should be discarded after this time).

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