← Antibiotics & Infections

Azithromycin

A long-acting macrolide antibiotic widely used for respiratory, skin, and sexually transmitted bacterial infections.

What is Azithromycin?

Azithromycin is a macrolide antibiotic that inhibits bacterial protein synthesis. It is particularly valued for its long half-life, excellent tissue penetration, and short treatment duration.

Because of these properties, azithromycin allows once-daily dosing and shorter courses compared to many other antibiotics.

History & Development

Azithromycin was developed as a semi-synthetic derivative of erythromycin. Structural modification of the macrolide ring improved stability, tissue distribution, and gastrointestinal tolerability.

Chemical Structure & Medicinal Chemistry

  • Molecular Formula: C₃₈H₇₂N₂O₁₂
  • Molar Mass: 748.98 g/mol
  • Drug Class: Macrolide antibiotic
  • Key Features: Large lactone ring with amino sugars

The expanded macrolide ring increases acid stability and enhances intracellular accumulation, contributing to its prolonged antimicrobial effect.

Mechanism of Action

Azithromycin inhibits bacterial protein synthesis by binding to the 50S ribosomal subunit.

  • Blocks translocation of peptide chains
  • Prevents bacterial protein synthesis
  • Primarily bacteriostatic in action

Selective toxicity arises because human ribosomes differ structurally from bacterial ribosomes.

Pharmacokinetics

  • Good oral absorption
  • Peak plasma levels in 2–3 hours
  • Extensive tissue penetration
  • Very long half-life (~68 hours)
  • Excreted mainly in bile

Clinical Uses

  • Community-acquired pneumonia
  • Acute bronchitis and sinusitis
  • Skin and soft tissue infections
  • Chlamydia trachomatis infections

Dosage Principles

Azithromycin is usually prescribed for short durations due to its prolonged tissue persistence.

Skipping doses or incomplete therapy increases the risk of antibiotic resistance.

Common Side Effects

  • Nausea and diarrhea
  • Abdominal discomfort
  • Headache

QT Prolongation & Cardiac Risk

Azithromycin may prolong the QT interval by affecting cardiac ion channels. This can increase the risk of serious arrhythmias in susceptible individuals.

Who Should Use Caution?

  • Patients with known QT prolongation
  • Individuals with cardiac arrhythmias
  • Patients taking other QT-prolonging drugs
  • Severe liver disease

Antibiotic Resistance

Inappropriate use of azithromycin contributes to macrolide resistance. Resistance mechanisms include ribosomal modification and reduced drug binding.

Drug Interactions

  • Other QT-prolonging medications
  • Antacids (reduce absorption)

Student & Exam Insight

Azithromycin is a classic macrolide antibiotic. Its mechanism, long half-life, and QT prolongation risk are frequently tested in pharmacology exams.

References

  • World Health Organization (WHO)
  • U.S. Food and Drug Administration (FDA)
  • PubChem Drug Database
  • National Center for Biotechnology Information (NCBI)

Written by Moinuddin Ahmed Pasha
Chemistry educator focused on simplifying pharmacology and medicinal science for students.

⚠️ This content is for educational purposes only and does not replace professional medical advice.