Antibiotic Therapy

Azithromycin

Macrolide (azalide) · bacteriostatic, protein-synthesis inhibitor (50S ribosome)

Species: Dogs & Cats9 dose protocols6 dosage forms

This page is a calculation and educational reference for veterinarians and veterinary students. It does not replace examination, culture and susceptibility testing, clinical judgment, or the attending veterinarian's final decision.

Drug overview

Macrolide (azalide) · bacteriostatic, protein-synthesis inhibitor (50S ribosome)

Brand names: Zithromax®

General dose: 5–10 mg/kg PO · once dailySource: Plumb's (general)

Spectrum of activity

A macrolide (azalide), bacteriostatic, with very high tissue penetration and a long half-life; its strength is intracellular/atypical and respiratory bacteria — Chlamydia, susceptible respiratory Mycoplasma species, Bordetella and Haemophilus — as well as Campylobacter, Helicobacter, streptococci and susceptible staph. Mycoplasma coverage is species-dependent and must not be generalized to all species. Activity against spirochetes (Borrelia) and intracellular Rhodococcus is organism-dependent. For selected nontuberculous mycobacteria, including some Mycobacterium avium complex isolates, azithromycin is only an organism-specific component of multidrug therapy after speciation and susceptibility assessment—not reliable monotherapy. But it is not reliable against Enterobacteriaceae (E. coli), Pseudomonas, enterococci and anaerobes. ⚠️ It is ineffective against Mycoplasma haemofelis (feline hemotropic mycoplasma).

Veterinary uses and doses

General (susceptible) infection

DogSource: Sykes 2003

5–10 mg/kg · once daily · 3–5 days

CatSource: Sykes 2003

Cat: 5–10 mg/kg · once daily · 3–5 days

Skin (pyoderma / dermatology)

DogSource: Merchant 2000

Dermatology: 5–10 mg/kg · once daily · 3–5 days

DogSource: Ramadinha 2002

Pyoderma: 10 mg/kg · once daily · 5–10 days

CatSource: Merck Veterinary Manual (2025)

Cat (dermatology): 7–15 mg/kg · every 24 h · 5–7 days

Upper respiratory infection (URI)

CatSource: ISCAID respiratory guideline 2017

Cat URI — reserve option: 5–10 mg/kg PO every 12 h on day 1, then every 72 h

Clinical note: Reserve azithromycin for cases in which doxycycline and amoxicillin are unsuitable and feline chlamydiosis is unlikely. Doxycycline remains first-line for suspected Chlamydia felis. Reassess clinical response after 7 days. For chronic disease, if continued after reassessment, treat for at least 7 days beyond clinical resolution or a stable clinical plateau; do not apply a routine fixed 6–8-week course.

Babesiosis (B. gibsoni)

DogSource: Birkenheuer 2004

10 mg/kg · once daily · 10 days

Combination therapy: with atovaquone 13.3 mg/kg every 8 h with a fatty meal; both drugs for 10 days

Pulse (extended-interval) dosing

DogSource: Aucoin 2002

5 mg/kg · days 1 and 2 daily, then one dose every 3–5 days (5 doses total)

CatSource: Aucoin 2002

Cat: 5 mg/kg · days 1 and 2 daily, then one dose every 3–5 days (5 doses total)

Dosage forms

Suspension 200 mg/5 mL
Suspension 100 mg/5 mL
Capsule 250 mg
Capsule 500 mg
Tablet 250 mg
Tablet 500 mg

Safety and clinical notes

Use with caution in hepatic impairment (metabolism and excretion are mainly hepatic-biliary).
The most common adverse effects are GI (vomiting, especially with high doses, and diarrhea); but less than with erythromycin.
Absorption is reduced by magnesium- or aluminium-containing antacids; separate by about 2 hours.
It may raise cyclosporine and digoxin levels; use with caution alongside them.
Follow the specific product label after reconstitution. For Zithromax oral suspension, store at 5–30°C (do not refrigerate), shake well, and use within 10 days.
It has a very long tissue half-life; this is what makes once-daily and pulse (extended-interval) regimens possible.
⚠️ It is ineffective against Mycoplasma haemofelis (feline hemotropic mycoplasma); use doxycycline for that.

Cited sources

Plumb's (general)
Sykes 2003
Merchant 2000
Ramadinha 2002
Merck Veterinary Manual (2025)
ISCAID respiratory guideline 2017
Birkenheuer 2004
Aucoin 2002

Calculate a weight-based dose

Drug-data last updated: 2026-07-28