Azithromycin (Zithromax®)

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  • Azithromycin, derived from erythromycin, belongs to the azalide subclass of macrolide antibiotics . Like other macrolide antibiotics, it works by binding to the "P" site of the 50S ribosomal subunit of susceptible microorganisms interrupting RNA-dependant protein synthesis.
  • Susceptible bacteria include: Staphylococcus aureus, various Streptococci, some Hemophilus spp, certain Bacteroides spp, Borrelia burgdorferi, some Mycoplasma spp, and some species of Chlamydia.
  • Following oral administration, azithromycin is rapidly absorbed and widely distributed throughout the body. Azithromycin's bioavailability is 97% in dogs, 58% in cats, and its protein binding is 7-51%, depending on the plasma concentration.
  • Rapid distribution of azithromycin into tissues results in significantly higher azithromycin levels in tissues than in plasma. The half-life of azithromycin is long. The tissue half-life in dogs may be up to 90 hours. In cats tissue the half-life is 13 to 72 hours, depending on the tissue. Azithromycin undergoes some hepatic metabolism but the majority of an administered dose is excreted unchanged in bile.
  • Azithromycin is a prescription drug and can only be obtained from a veterinarian or by prescription from a veterinarian.
  • This drug is not approved for use in animals by the Food and Drug Administration but it is prescribed legally by veterinarians as an extra-label drug.

    Brand Names and Other Names

  • This drug is registered for use in humans only.
  • Human formulations: Zithromax® (Pfizer)
  • Veterinary formulations: None

    Uses of Azithromycin

    Treatment of infections caused by susceptible microorganisms:

  • Dermatological infections
  • Respiratory tract infections
  • Urogenital infections
  • Otitis media

    Precautions and Side Effects

  • Azithromycin should not be used in animals with known hypersensitivity or allergy to it or other macrolide antibiotics.
  • In addition, it should be used with great caution (if at all) when there is preexisting liver disease.
  • Gastrointestinal side effects may include vomiting, diarrhea, or abdominal pain.
  • Angioedema and cholestatic jaundice have been reported (rarely) in treated humans.
  • Cardiac arrhythmias, including ventricular tachycardia, may be precipitated by azithromycin.
  • Renal dysfunction, including interstitial nephritis and acute renal failure, may occur secondary to azithromycin treatment and liver function may be affected.

    Drug Interactions

  • Azithromycin may elevate serum digoxin levels.
  • When ergotamine or dihydroergotamine are concurrently administered with azithromycin ergot toxicity may occur.
  • Azithromycin causes a decrease in the clearance of triazolam and thus an increase in its pharmacologic effects
  • Pimozide is contraindicated in patients receiving azithromycin, and vice versa (death may result)
  • Animals being treated with cisapride should not be given azithromycin or other macrolide antibiotic
  • Drugs metabolized by cytochrome P450 (e.g. carbamazepine, terfenadine, cyclosporine, hexobarbital, and phenytoin) will have their serum levels elevated by azithromycin
  • Oral antacids reduce the absorption of azithromycin

    How Azithromycin is Supplied

  • Tablets: 250 mg, 500 mg, 600 mg
  • Powder for injection: 500 mg (lyophilized) in 10 mL vials
  • Powder for oral suspension: 100 mg/5 mL, 200 mg/5mL, & 1 g/packet

    Dosing Information

  • Medication should never be administered without first consulting your veterinarian.
  • In dogs, the usual dosage is 2.5 to 5 mg per pound (5 to 10 mg/kg) orally once daily for up to 7 days
  • In cats, the usual dosage is 2.5 to 7.5 mg per pound (5 to 15 mg/kg) orally every 12 to 24 hours for up to 7 days
  • The duration of administration depends on the condition being treated, response to the medication and the development of any adverse effects.
  • Be certain to complete the prescription unless specifically directed by your veterinarian. Even if your pet feels better, the entire treatment plan should be completed to prevent relapse.

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