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Azithromycin is a macrolide antibiotic that most cats take every three days rather than every day. The usual dose is 5 to 10 mg per kilogram, and that unusual schedule is the reason vets reach for it. The drug parks itself in tissue and keeps working long after it has cleared the bloodstream.
It's a useful antibiotic and I do prescribe it. It's also handed out more often than it should be for cat colds.
Key Takeaways
The usual feline dose is 5 to 10 mg per kilogram of body weight by mouth. What changes between conditions is the schedule, not the amount.
| What's being treated | Dose | Schedule |
|---|---|---|
| Chronic upper respiratory infection | 5 to 10 mg/kg by mouth | Twice on day 1, then once every 3 days1 |
| Cytauxzoonosis, given with atovaquone | 10 mg/kg by mouth | Once daily for 10 days4,5 |
| Illness caused by Bartonella | 10 mg/kg by mouth | Daily for 7 days, then every other day6 |
| Palliative care | 5 to 10 mg/kg by mouth | Once every 3 days, continued indefinitely |
In palliative care cases I give azithromycin every three days indefinitely, so there's no course to finish.
It comes as tablets and as an oral suspension. The suspension is easier to dose accurately for a small cat, and most cats take it more readily than a tablet.
Follow the schedule on your own bottle rather than the table above. A cat on a three day schedule is easy to lose track of, so I'd suggest putting the dosing days in your phone the day you pick up the prescription. Missed doses are the most common reason a course doesn't work.
The drug binds to the 50S ribosome inside bacteria and stops them making protein.3 That halts growth rather than killing the bacteria outright, so it's described as bacteriostatic. Your cat's immune system does the finishing.
About 58% of an oral dose makes it into a cat's bloodstream.2 That's lower than in dogs, where it's closer to 97%, which is why feline dosing isn't simply scaled down from canine dosing.
From there it leaves the blood quickly and concentrates in tissue, including inside the white blood cells that travel to a site of infection.3 Tissue levels end up many times higher than blood levels and stay up for days. That's what allows the every third day schedule. It also means a blood level would tell you very little about whether the drug is working.
It clears slowly and leaves the body mostly unchanged in the bile rather than being broken down by the liver.3
Less often than it gets prescribed. This is the conversation I have most often in the exam room.
Most feline upper respiratory infections are viral. Antibiotics do nothing for a virus, and the ISCAID guidelines are direct about it. Clear, watery nasal discharge doesn't warrant an antibiotic at all.1
Thick coloured discharge in a cat who is otherwise bright and eating doesn't either, at least not straight away. The guideline is to wait and watch for up to 10 days. What changes the answer is fever, lethargy, or a cat that has gone off food alongside that discharge.1
When a cat does need one, doxycycline is the first choice at 5 mg/kg twice daily or 10 mg/kg once daily for 7 to 10 days. It's well tolerated by cats and it covers Bordetella bronchiseptica, Chlamydia felis and Mycoplasma.1
Azithromycin sits behind it. ISCAID notes it isn't as effective as doxycycline against feline ocular chlamydiosis and advises keeping it for situations where Chlamydia is unlikely to be the cause.1 It earns its place with the cat who won't take doxycycline, or the household where a dose every three days is the difference between a course that gets finished and one that doesn't.
Not every cat with a runny nose has an infection. Chronic one-sided discharge in particular can be a nasal polyp, which no antibiotic will fix.
The strongest evidence for azithromycin in cats isn't respiratory at all. It's cytauxzoonosis, a tick-borne parasite that is usually fatal without treatment. Paired with atovaquone at 15 mg/kg every 8 hours, azithromycin at 10 mg/kg daily for 10 days lifts survival to around 60% when it's started early and backed by intensive supportive care.4,5 That's the one setting where this drug clearly changes the outcome.
It also has a role in toxoplasmosis, usually as an alternative when clindamycin isn't suitable.
Bartonella is where I'd be most cautious. Azithromycin does appear in the ABCD treatment table at 10 mg/kg, but every option listed there rests on case reports rather than controlled trials, and treatment is reserved for the uncommon cat in which Bartonella has actually caused disease, such as endocarditis. Treating a healthy carrier isn't recommended. Clearing the bacteria can't be guaranteed, and a failed course risks selecting for resistance.6
You may also see azithromycin mentioned as a gut motility drug. The macrolides do have that effect, but the feline evidence is thin, and cisapride is the drug I'd reach for first in a cat with poor motility or ongoing constipation.
Most cats tolerate it well. When something does go wrong, it's almost always the stomach.
Vomiting is the one I see most, followed by diarrhea and a drop in appetite. These are usually mild and often settle on their own. Giving the dose with a small amount of food helps. If your cat is vomiting after every dose, or has stopped eating altogether, stop and call rather than pushing through the course. Appetite that doesn't come back within a day or two needs a look, because cats that stop eating get into trouble faster than dogs do.
Facial swelling or hives point to an allergic reaction and need to be seen the same day. Liver injury and changes in white blood cell counts have both been reported, and both are uncommon.
Because the drug hangs around in tissue for days, a side effect doesn't disappear the moment you stop giving it. Give it a few days before you assume something else is going on.
Tell your vet if your cat has liver disease. Azithromycin is cleared mostly in the bile,3 so a cat with reduced liver function may clear it more slowly. That's a reason for caution and monitoring rather than an automatic no, so raise it if your cat has a history of jaundice or abnormal liver values.
Avoid it in any cat that has reacted to a macrolide before, meaning azithromycin itself, erythromycin or clarithromycin. Clindamycin is a lincosamide rather than a macrolide, but the two classes share a binding site and cross-resistance between them is well described, so mention a past clindamycin reaction as well.
Antacids can interfere with absorption, and phenobarbital can affect how the drug is handled, so run your cat's full medication list past your vet before starting.
Resistance is the one I'd weigh most heavily. Every course of an antibiotic that wasn't needed makes the next infection harder to treat, and macrolide resistance is already common. A cat with a clear runny nose and a good appetite is usually better served by a few days of watching than by a prescription.
If your cat has been prescribed azithromycin and you're not sure how to time the doses, or you're wondering whether an antibiotic is the right call at all, give us a call at (519) 896-0532.
The usual dose is 5 to 10 mg per kilogram of body weight by mouth. What changes between conditions is the schedule, not the amount. For a chronic upper respiratory infection, the ISCAID guidelines give it twice on day one and then once every three days. For cytauxzoonosis it's 10 mg/kg once daily for 10 days alongside atovaquone. For illness caused by Bartonella it's 10 mg/kg daily for 7 days and then every other day. In palliative care cases I give it every three days indefinitely. Follow the schedule on your own bottle, because your vet may have adjusted it for your cat.
Often it doesn't. Most feline upper respiratory infections are viral, and antibiotics do nothing for a virus. The ISCAID guidelines don't recommend treating when the nasal discharge is clear and watery. If the discharge is thick and coloured but your cat is bright and eating normally, the guideline is to wait and watch for up to 10 days rather than prescribe straight away. What changes the answer is fever, lethargy, or a cat that has gone off food alongside that discharge. When a cat does need an antibiotic, doxycycline is the first choice rather than azithromycin.
Most cats improve within 24 to 48 hours. That's a sign the drug is working, not a sign the infection is cleared, so finish the full course even once your cat seems back to normal. Cutting a course short leaves behind the bacteria that were hardest to kill, which is how resistance builds. If there's no improvement at all after two to three days, call your vet rather than continuing. It may be the wrong drug for what your cat has, or the problem may not be bacterial.
Give it as soon as you remember, then restart the interval from that point. If you're close to the next scheduled dose, skip the missed one and carry on as normal. Never give two doses together to catch up. This drug is easy to lose track of because many cats are on a three day schedule rather than a daily one, so it helps to set a repeating reminder on your phone the day you pick up the prescription.
Yes. Antacids can reduce how much of the drug your cat absorbs, and phenobarbital can change how it's handled. Azithromycin leaves the body mostly unchanged in the bile rather than being broken down by the liver, but a cat with reduced liver function may still clear it more slowly, so mention any history of liver disease or jaundice. Avoid it entirely in a cat that has reacted to a macrolide before, meaning azithromycin, erythromycin or clarithromycin. Give your vet the full list of everything your cat is taking, including supplements, before you start.
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