Is azithromycin (Zithromax) the same as erythromycin (Erythrocin) ethylsuccinate?

Azithromycin and Erythromycin Ethylsuccinate Are Different Medications

No, azithromycin (Zithromax) is not the same as erythromycin ethylsuccinate (Erythrocin). While both belong to the macrolide antibiotic class, they are distinct medications with different chemical structures, dosing regimens, side effect profiles, and clinical applications.

Key Differences Between Azithromycin and Erythromycin Ethylsuccinate

Chemical Structure

• Azithromycin is an azalide, a subclass of macrolide antibiotics derived from erythromycin, but with a methyl-substituted nitrogen atom incorporated into the lactone ring, making it a 15-membered ring structure 1, 2
• Erythromycin ethylsuccinate is an ester form of erythromycin base with a 14-membered ring structure 3

Dosing Regimen

• Azithromycin: Typically administered once daily for 5 days (500 mg on day 1, followed by 250 mg on days 2-5) for most infections 3
• Erythromycin ethylsuccinate: Requires administration four times daily for 7-14 days (typically 400-800 mg four times daily) 3

Pharmacokinetics

• Azithromycin has:

  • Longer half-life (approximately 3 days in tissues)
  • Higher tissue concentrations
  • Lower serum concentrations
  • Better tissue penetration
  • Simpler dosing schedule 4, 5

• Erythromycin ethylsuccinate has:

  • Shorter half-life
  • Lower tissue penetration
  • More frequent dosing requirements 4, 6, 7

Side Effect Profile

• Azithromycin has fewer gastrointestinal side effects compared to erythromycin ethylsuccinate 4, 5
• Erythromycin ethylsuccinate causes more gastrointestinal irritation including epigastric distress, abdominal cramps, nausea, vomiting, and diarrhea 3, 6

Drug Interactions

• Erythromycin ethylsuccinate has more significant drug interactions as it inhibits the cytochrome P450 enzyme system 3, 8
• Azithromycin has fewer drug interactions with medications metabolized by the cytochrome P450 system 5

Clinical Preference

• Azithromycin is often preferred due to:
  • Better compliance (shorter duration, once-daily dosing)
  • Fewer side effects
  • Fewer drug interactions
  • Better activity against certain pathogens like Haemophilus influenzae 8, 5

Clinical Applications Where They Differ

Treatment of Pertussis

• For infants <1 month: Azithromycin is preferred over erythromycin due to risk of infantile hypertrophic pyloric stenosis (IHPS) with erythromycin 3
• For older patients: Azithromycin requires 5 days of treatment versus 14 days for erythromycin ethylsuccinate 3

Treatment of Sexually Transmitted Infections

• Azithromycin can be given as a single 1g dose for chlamydial infections 3
• Erythromycin ethylsuccinate requires 7 days of therapy (800 mg four times daily) 3

Common Pitfalls and Caveats

1. Dosing confusion: Don't assume similar dosing between these medications. Erythromycin ethylsuccinate requires four times daily dosing while azithromycin is once daily.

2. Duration confusion: Erythromycin ethylsuccinate typically requires longer treatment courses (7-14 days) compared to azithromycin (3-5 days).

3. Drug interaction awareness: Erythromycin has significant interactions with medications metabolized by cytochrome P450 enzymes, while azithromycin has fewer such interactions.

4. Pregnancy considerations: Both azithromycin and erythromycin are FDA Pregnancy Category B drugs, but erythromycin estolate (a different salt form) is contraindicated during pregnancy due to hepatotoxicity 3.

5. Infant safety: Erythromycin use in infants under 1 month is associated with infantile hypertrophic pyloric stenosis (IHPS), making azithromycin preferred in this population 3.

In conclusion, while both medications are macrolide antibiotics, they have significant differences in structure, pharmacokinetics, dosing, side effects, and clinical applications that make them distinct therapeutic agents.