Azithromycin
Azithromycin dihydrate
Also known as: Zithromax · Zmax · Azyter · Clamelle · Zedbac · azithromycinum · acitromicina · CP-62993 · XZ-450
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Audited v13 dosing evidence
Structured calculator evidenceGI promotility (extra-label)
Verified clinician required
- q8h
NA
Governing clinical context (3)
- Intact injectable product should be stored at room temperature below 30°C (86°F). After reconstitution with sterile water for injection, solutions containing 100 mg/mL will be stable for 24 hours if stored below 30°C (86°F). Further diluted solution of 2 mg/mL or less will remain physically and chemically stable for 24 hours at room temperature and up to 7 days if refrigerated at 5°C (41°F).
- ■ Oral suspension should be shaken well before each use. Store the suspension at room temperature (do not refrigerate). It is preferable to give suspension when the animal has an empty stomach.
- Commercially available tablets should be stored at room temperature less than 30°C (86°F). Oral suspension products for reconstitution should be stored at room temperatures between 5°C to 30°C (41°F-86°F) before reconstitution. After reconstitution, the multiple-dose product may be stored at room temperatures between 5°C to 30°C (41°F-86°F) for up to 10 days, then discarded. The single- dose packets should be given immediately after reconstitution.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Azithromycin is a broad-spectrum, semi-synthetic macrolide antibiotic belonging to the azalide subclass. It is widely used in veterinary medicine due to its highly favorable pharmacokinetic profile, which allows for less frequent dosing compared to older macrolides like erythromycin.
Key Clinical Features:
- Exceptional Tissue Penetration: Azithromycin is actively taken up by phagocytes (macrophages and polymorphonuclear leukocytes) and fibroblasts, which then migrate to sites of infection. This results in tissue concentrations that can be up to 100 times higher than serum concentrations.
- Long Half-Life: The drug exhibits a very long tissue half-life (up to 90 hours in dogs), allowing for pulse-dosing regimens (e.g., every 3 to 5 days) after an initial loading phase.
- Spectrum of Activity: Effective against a variety of Gram-positive organisms (e.g., Streptococcus, Staphylococcus), some Gram-negative organisms (Haemophilus, Bordetella), and atypical pathogens including Mycoplasma pneumoniae, Borrelia burgdorferi, Toxoplasma spp., and Bartonella spp.
- Specific Uses: Frequently utilized for canine Bordetella pneumonia, feline upper respiratory infections, feline Bartonellosis, Babesia gibsoni in dogs (in combination with atovaquone), and Rhodococcus equi infections in foals.
Clinical Pearl: Despite its broad utility, azithromycin has been shown to be ineffective for clearing Chlamydophila felis or Mycoplasma haemofelis in cats.
Mechanism of action
Azithromycin exerts its antibacterial effects by binding to the 50S ribosomal subunit of susceptible microorganisms.
- Mechanism: By binding to the 50S subunit, it blocks transpeptidation and translocation processes → inhibits RNA-dependent protein synthesis.
- Action Type: It is generally considered bacteriostatic, but can be bactericidal at high concentrations or against highly susceptible organisms.
- Targeted Delivery: The drug is actively transported to the site of infection by host phagocytic cells. During active phagocytosis, azithromycin is released locally, providing high concentrations directly at the site of bacterial invasion.
Safety & warnings
Contraindications
- Hypersensitivity to azithromycin or any other macrolide antibiotics
Adverse effects
- Vomiting (especially at high doses)
- GI cramping
- Potential hepatotoxicity
- Local tissue reactions (IV site)
- Muscle damage (IM injection)
- Diarrhea in foals
Precautions
Hepatic Impairment: Because azithromycin is primarily eliminated via the liver (excreted unchanged in the bile), it should be used with caution in patients with impaired hepatic function.
Pregnancy: Safety during pregnancy has not been fully established in veterinary species. Use only when clearly necessary. In humans, it is an FDA Category B drug (animal studies have not demonstrated fetal risk, but adequate human studies are lacking).
Injection Site: IM injection is generally avoided in small animals and rabbits due to a high degree of muscle damage at the injection site.
Drug interactions
May reduce the rate of absorption of azithromycin; separate dosages by 2 hours
Other macrolides are contraindicated with cisapride due to potential for fatal arrhythmias; use with extreme caution or avoid
Azithromycin may potentially increase cyclosporine blood levels; monitor carefully
Other macrolides can increase digoxin levels; monitor carefully
Contraindicated; acute deaths have occurred in humans
May increase serum levels of methylprednisolone
May increase serum levels of theophylline
May increase serum levels of terfenadine
Monitoring
- Clinical efficacy (resolution of infection signs)
- Adverse effects (especially GI distress or signs of hepatic impairment)
Pharmacokinetics
Half-life
Absorption
Dogs: 97% oral bioavailability. Cats: 58% oral bioavailability. Foals: 40-60% oral bioavailability. Adult horses: 1-7% oral bioavailability.
Distribution
Extensive tissue distribution. Tissue concentrations do not mirror serum concentrations and are significantly higher. Concentrates in bronchoalveolar cells, pulmonary epithelial fluid, and white blood cells. Foals Vd: 11.6-18.6 L/kg. Goats/Sheep Vd: 34.5 L/kg.
Metabolism
Hepatic.
Elimination
Primarily excreted unchanged in the bile (>50% of an oral dose in dogs and cats).
Overdose
Acute oral overdoses are unlikely to cause significant morbidity other than gastrointestinal distress, including vomiting, diarrhea, and GI cramping. Treatment should be supportive and symptomatic.
Available products
Formulations
- Tablets
- Powder for oral suspension
- Powder for injection
Veterinary
- None commercially available. Preparations compounded for dogs and cats may be available from compounding pharmacies.
Human-labeled
- Azithromycin Oral Tablets: 250 mg, 500 mg & 600 mg (as dihydrate) (Zithromax, generic)
- Azithromycin Powder for Oral Suspension: 100 mg/5 mL, 200 mg/5 mL, 1 g/packet, and 2 g extended-release single-dose bottles (Zithromax, Zmax, generic)
- Azithromycin Powder for Injection (lyophilized): 500 mg in 10 mL vials (Zithromax, generic)
Regulatory status
POM (Prescription Only Medicine)
POM-V or POM (Prescription Only Medicine)
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Tablets: Store below 30°C. Powder for oral suspension: Store between 5-30°C before reconstitution. After reconstitution, multiple-dose products may be stored between 5-30°C for up to 10 days and then discarded. Single-dose packets should be given immediately after reconstitution. Injectable: Store below 30°C; reconstituted solutions (100 mg/mL) are stable for 24 hours below 30°C.
Client information
Administration Tips:
- Give the medication exactly as prescribed by your veterinarian. Because this drug stays in the body for a long time, your vet may instruct you to give it only every few days. Do not skip doses or stop early, even if your pet seems better.
- If using the liquid suspension, shake it well before each use. It is best given on an empty stomach, but if it causes stomach upset, you may give it with a small treat or meal.
- Do not refrigerate the oral suspension.
- Discard any unused oral suspension after 14 days.
What to Watch For:
- Azithromycin is generally well-tolerated, but it can cause stomach upset.
- Contact your veterinarian immediately if your pet develops severe diarrhea, persistent vomiting, loss of appetite, or if their condition seems to be getting worse.
- For rabbits and guinea pigs: Stop the medication and call your vet immediately if you notice soft stools, diarrhea, or a decrease in appetite.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.