How Zithromax Is Used as an Antibiotic

Zithromax is a versatile antibiotic used to treat many types of infections, including skin infections, ear infections, respiratory infections, and sexually transmitted infections. It is available as a pill or injection.

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Mechanism of Action

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

This small difference tweaks the bacterial coverage of Zithromax and curiously changes its route of metabolism. Whereas erythromycin is metabolized by the cytochrome P450 system, Zithromax isn't. Thus, Zithromax interacts with fewer drugs, unlike erythromycin, which has a drug-drug interaction with some statins (e.g., Zocor or Crestor).

Like the other macrolides—erythromycin and clarithromycin—Zithromax works by binding to the bacteria 50S ribosomal subunit, thus interfering with the bacteria's ability to produce proteins. Bacteria need to produce proteins to survive. (Depending on the organism and the drug concentration, macrolides can be either bacteriostatic [stopping bacterial growth] or bactericidal [killing bacteria].)


Here are some of the bacteria that Zithromax fights:

  • Mycobacterium avium complex (MAC)
  • Toxoplasmosis gondii
  • Haemophilus influenza
  • Streptococcus and staphylococcus (slightly less active than erythromycin)
  • Helicobacter
  • Moraxella catarrhalis
  • Borrelia burgdorferi
  • Chlamydia
  • Leginoella
  • Chlamydia
  • Mycoplasma
  • Mycobacterium avium complex
  • Haemophilus
  • Streptococcus and staphylococcus
  • Moraxella catarrhalis

Clinical uses for Zithromax include:

  • 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 Zithromax a poor choice for treating certain infections like community-acquired pneumonia, otitis media (ear infection), and acute sinusitis. Of particular note, Zithromax is ineffective in combating infections caused by MRSA, a superbug with broad antibacterial resistance.

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

Although unable to cross the blood-brain barrier (and treat meningitis), Zithromax deeply pervades our tissue. This drug is also slowly released into systemic circulation and has a half-life of about 68 hours. These amenable pharmacokinetic properties enable clinicians to administer the drug more infrequently. (Half-life refers to the time it takes for the concentration of drug in the blood to decrease by half. If the half-life is 48 hours, then 48 hours after administration, half the drug will have cleared from the blood.)

With respect to cervicitis and urethritis caused by infection with chlamydia, a single dose (injection) of Zithromax is equally effective as a seven-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 Zithromycin and at what dosage. Please discuss such treatment with your physician.

Adverse Effects

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.

A Word from Verywell

If you or a loved one has heart disease, it may be a good idea to avoid Zithromycin. This drug is known to cause QT-prolongation, arrhythmia, and sudden death. Of note, QT-prolongation interferes with heart rhythms.

The nice thing about Zithromycin is that its long half-life and fewer doses limit nonadherence. For example, one dose of this drug is a complete treatment; whereas, if you must take doxycycline on your own for a week, you may forget or stop.

Nevertheless, when prescribed Zithromycin for respiratory infections, throat infection and so forth, it's incumbent on us to see the 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. 

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5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Fohner AE, Sparreboom A, Altman RB, Klein TE. PharmGKB summary: Macrolide antibiotic pathway, pharmacokinetics/pharmacodynamicsPharmacogenet Genomics. 2017;27(4):164-167. doi:10.1097/FPC.0000000000000270

  2. U.S. Food and Drug Administration. Zithromax Label.

  3. Fair RJ, Tor Y. Antibiotics and bacterial resistance in the 21st centuryPerspect Medicin Chem. 2014;6:25–64. Published 2014 Aug 28. doi:10.4137/PMC.S14459

  4. Centers for Disease Control and Prevention. 2015 Sexually Transmitted Diseases Treatment Guidelines. Updated December 27, 2019.

  5. Juurlink DN. The cardiovascular safety of azithromycinCMAJ. 2014;186(15):1127–1128. doi:10.1503/cmaj.140572

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