Macrolide

Erythromycin

A natural macrolide for chest and skin infections, whooping cough, chlamydia and acne, often when penicillin cannot be used.

Drug card

Class: Macrolide
How it works: Binding the 50S ribosomal subunit so the growing protein chain cannot move on
Used for: Chest infections, including Mycoplasma and Legionella
Strep throat in penicillin allergy
Skin infections
Whooping cough (pertussis)
Known for: Nausea and vomiting
QT prolongation and torsades de pointes
Its name: -mycin means an antibiotic from Streptomyces bacteria

Study it as a flashcard, and it comes back before you forget it.

Mechanism of action

How does erythromycin work?

Erythromycin is made by Saccharopolyspora erythraea. It binds the 50S ribosomal subunit and blocks protein synthesis, and is bacteriostatic or bactericidal depending on its concentration and the organism. It is cleared mainly by the liver and is both a substrate and an inhibitor of CYP3A4.

Like other macrolides: Macrolides are large lactone-ring antibiotics that bind the 23S rRNA of the bacterial 50S ribosomal subunit and block translocation, so the growing peptide chain cannot move on and protein synthesis stalls. They are mainly bacteriostatic. Bacteria resist by methylating the 23S rRNA, which also defeats lincosamides, or by pumping the drug out.

Uses

What is erythromycin used for?

Chest infections, including Mycoplasma and Legionella
Strep throat in penicillin allergy
Skin infections
Whooping cough (pertussis)
Chlamydia in pregnancy and in newborns
Acne

Side effects

Side effects of erythromycin

Nausea and vomitingKnown for
QT prolongation and torsades de pointesKnown forSerious
Abdominal pain and cramps
Diarrhea
Loss of appetite
Cholestatic hepatitis and jaundiceSerious
Pyloric stenosis in young infantsSerious
Reversible hearing loss at high dosesSerious
Worsening of myasthenia gravisSerious
Allergic reactions, from hives to anaphylaxisSerious
PancreatitisSerious
Interstitial nephritisSerious

Known for marks what an exam expects you to know it for.

Watch for

Cautions and what sets it apart

Inhibits CYP3A4: never with simvastatin, lovastatin, pimozide or cisapride; colchicine toxicity and rhabdomyolysis have occurred.
QT risk: avoid with long QT, low potassium or magnesium, bradycardia, or class IA or III antiarrhythmics.
It raises theophylline and digoxin levels and the effect of warfarin, so levels or the INR are checked.
Syphilis in pregnancy: it does not reach the fetus well, so the baby still needs penicillin treatment.

Good to know: Azithromycin is derived from it. In young infants, treated for whooping cough or chlamydia, the risk of pyloric stenosis is weighed against the danger of the infection.

Remember it

A way to remember it

MACRO

M: Myasthenia gravis can worsen
A: Arrhythmia from QT prolongation (torsades de pointes)
C: Cholestatic jaundice
R: Reversible hearing loss
O: Other drugs' levels rise: clarithromycin and erythromycin inhibit CYP3A4

Think MACRO for the macrolide warnings. Stomach upset (nausea, vomiting, diarrhea) is the commonest complaint of all.

Written for biofideX

Its name

-mycin means antibiotic from Streptomyces bacteria
Spans classes: macrolides (azithromycin), lincosamides (clindamycin), glycopeptides (vancomycin), aminoglycosides (streptomycin) and anticancer bleomycin. Gentamicin is spelled -micin.
Every drug name stem

Its class

Macrolides

Other macrolides with a card:

Easily mixed up with lincosamides, tetracyclines and fluoroquinolones.

All about macrolides

Sources

Checked against

Written for biofideX in plain words, after checking these references:

For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.

Practice it

Antibiotics: protein synthesis

Erythromycin is one of 10 drugs practiced together here: their classes, how they work, what they treat and what they're known for.

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