What Does the Antibiotic Azithromycin Do?

Azithromycin treats many types of bacterial infections.

Molecular Structure of Azithromycin. Getty Images

The antibiotic azithromycin is derived from erythromycin, another type of antibiotic. Although both "macrolides," technically azithromycin is an azalide, and erythromycin is a macrolide. In chemical terms, azithromycin shares the exact same lactose-macrolide structure as erythromycin save for the injection of a methylated nitrogen in the lactone ring.  

This small difference tweaks the bacterial coverage of azithromycin and curiously changes its route of metabolism.

Whereas erythromycin is metabolized by the cytochrome P450 system, azithromycin isn't. Thus, azithromycin interacts with few drugs, and unlike erythromycin, won't have a drug-drug interaction with a statin (think Zocor or Crestor).

Like the other macrolides—erythromycin and clarithromycin—azithromycin works by binding to the bacteria 50S ribosomal subunit; thus, crippling bacteria's ability to produce proteins. Bacteria need to produce proteins to survive. (Depending on the dosage, macrolides can be either bacteriostatic or bacteriocidal.)

Here are some of the bacteria that azithromycin fights:

  • Mycobacterium Avium Complex (MAC)
  • Toxoplasmosis gondii
  • Haemophilus influenza
  • streptococcus and staphylococcus (slightly less active than erythromycin)
  • helicobacter
  • Moraxella catarrhalis
  • Borrelia burgdorferi
  • Chlamydia

Here are some clinical uses for azithromycin:

  • respiratory tract infections
  • skin and other soft-tissue infections
  • acute bacterial exacerbations of COPD
  • otitis media
  • community-acquired pneumonia
  • bacterial conjunctivitis (eye infection)
  • Mycobacterium avium complex or MAC (an opportunistic respiratory infection common with AIDS)
  • chancroid
  • pharyngitis
  • tonsillitis
  • chlamydial cervicitis and urethritis (sexually transmitted infection)

    It should be noted that high levels of antibiotic resistance make azithromycin a poor choice for treating certain infections like community-acquired pneumonia, otitis media (ear infection) and acute sinusitis. Of particular note, azithromycin is ineffective in combating infections caused by MRSA, a superbug with broad antibacterial resistance.

    Depending on the route of administration, azithromycin is available in tablets, oral suspensions, and ophthalmic solutions.

    Although unable to cross the blood-brain barrier (and treat meningitis), azithromycin deeply penetrates our tissue. This drug is also slowly released into systemic circulation and has a half-life of about 48 hours. These amenable pharmacokinetic properties enable clinicians to administer the drug more infrequently.

    With respect to cervicitis and urethritis caused by infection with chlamydia, a single one-gram dose (injection) of azithromycin is equally effective as a 7-day course of doxycycline, therefore, limiting medication nonadherence.

      (The CDC recommends that clinicians treat gonorrhea and chlamydia together, so-called "dual" therapy.  Therefore, a clinician typically prescribes a shot of the cephalosporin Rocephin to treat for possible gonorrhea, too.) Please note that it's at your physician's discretion whether to treat you with azithromycin and at what dosage. If interested, please discuss such treatment with your physician.

    Although not as severe as erythromycin, azithromycin can also cause gastrointestinal distress like nausea or vomiting. These unwanted side effects can be mitigated by eating some food before you take an oral dose of azithromycin.

    If you or a loved one has heart disease, it may be a good idea to avoid azithromycin. Azithromycin is known to cause QT-prolongation, arrhythmia, and sudden death.

    The nice thing about azithromycin is that its long half-life and fewer doses limit nonadherence.  For example, one 2 g dose of azithromycin taken in a physician's office is a complete treatment; whereas, if you must take doxycycline on your own for a week, you may forget or quit.

    Nevertheless, when prescribed azithromycin for respiratory infections, throat infection and so forth, it's incumbent on us to see treatment through. When we fail to complete treatment and complete killing bacteria in our body, resistant bacteria survives and spreads--infecting others, recombining and conferring resistance to other bacteria. Please remember that by skipping out on antibiotic treatment, we contribute to antibiotic resistance, which is a global problem. The battle against antibiotic resistance is one of continuous ebb and flow, with many of our earlier medical victories washed away. We must all do our best to combat antibiotic resistance. 


    Deck DH, Winston LG. Chapter 44. Tetracyclines, Macrolides, Clindamycin, Chloramphenicol, Streptogramins, & Oxazolidinones. In: Katzung BG, Masters SB, Trevor AJ. eds. Basic & Clinical Pharmacology, 12e. New York, NY: McGraw-Hill; 2012.

    Mosby's Drug Reference for Health Professionals, Second Edition published by Elsevier in 2010.

    O'Donnell MR, Saukkonen JJ. Chapter 168. Antimycobacterial Agents. In: Longo DL, Fauci AS, Kasper DL, Hauser SL, Jameson J, Loscalzo J. eds. Harrison's Principles of Internal Medicine, 18e. New York, NY: McGraw-Hill; 2012.

    Continue Reading