DurgicalDurgical
EN
Open the app

Pharmacology · Macrolides

Erythromycin !

The first macrolide. Still used as an eye ointment for newborns and as an alternative to penicillin; it also stirs the gut via motilin receptors.

Updated 2026-11-01

  • Class: Macrolides
  • Target: Bacterial ribosome, 50S subunit (23S rRNA, exit tunnel)
  • ATC code: J01FA01
  • WHO Model List of Essential Medicines

The figure shows erythromycin bound in the exit tunnel of the Escherichia coli ribosome.

Target in 3D: Bacterial ribosome, 50S subunit (23S rRNA, exit tunnel). PDB structure 4V7U (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Erythromycin binds the 23S rRNA in the exit tunnel of the 50S subunit and stops the growing protein chain. It is unstable in stomach acid, so oral forms are protected salts or esters. It also binds and activates the motilin receptor of the gut, which explains its cramps and diarrhoea, and it inhibits CYP3A4.

  1. Reaches the bacterium after absorption or local application
  2. Binds the 23S rRNA in the 50S exit tunnel
  3. The nascent protein cannot pass
  4. Bacterial growth stops

Pharmacokinetics

Absorption
Destroyed by stomach acid; oral products are coated or given as salts and esters. IV form (lactobionate) and eye ointment exist.
Metabolism
Liver; it inhibits CYP3A4.
Elimination
Concentrated in the liver and excreted in the bile.

Indications

Adverse effects

Common

  • Abdominal cramps, nausea, vomiting and diarrhoea
  • Pain and vein irritation with the IV form
  • Rash

Contraindications

  • Allergy to erythromycin
  • Use with terfenadine, astemizole, cisapride, pimozide, ergotamine or dihydroergotamine
  • Use with lovastatin or simvastatin

Key interactions

Simvastatin and lovastatin
Higher risk of myopathy and rhabdomyolysis (CYP3A4).
Pimozide, cisapride and other QT drugs
Dangerous arrhythmias.
Other CYP3A4 substrates (e.g. carbamazepine, ciclosporin)
Their levels rise.

What is monitored

  • ECG with IV use or QT risk
  • Hearing with high doses in older patients
  • Liver tests if symptoms; vomiting in infants

Administration and safety

  • The IV form is infused slowly and diluted, preferably as a continuous or intermittent infusion, to limit vein pain
  • Never given as an IV push
  • The eye ointment is applied once to both eyes of the newborn

Nursing care

  • Check the IV site for pain and phlebitis
  • Warn about cramps and nausea
  • Review the medication list for statins and QT drugs
  • In infants, ask the family to report vomiting after feeds

Pregnancy and breastfeeding

Crosses the placenta and reaches breast milk; in pregnancy it is used when the benefit justifies the risk.

Test yourself

Think of the answer, then tap the question to see it.

Why does erythromycin cause abdominal cramps?

It activates the motilin receptor in the gut and increases gut motility.

Which newborn use does erythromycin eye ointment have?

Prevention of ophthalmia neonatorum caused by gonococcus or chlamydia.

Which serious infant problem has been linked to erythromycin?

Infantile hypertrophic pyloric stenosis.

Add to my cards (3)

← Back to the class: Macrolides

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - ERYTHROMYCIN LACTOBIONATE injection, powder, for solution (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - ERYTHROMYCIN ointment (DailyMed (U.S. National Library of Medicine), 2026); Erythromycin: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2026); Cryo-EM structure of the erythromycin-bound motilin receptor-Gq protein complex (RCSB Protein Data Bank, 2026).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.