Antimicrobials/Macrolide

Macrolides

also: erythromycin · clarithromycin · azithromycin

Overview

The main penicillin-allergy alternative for Gram-positive/atypical cover. QT prolongation and CYP3A4 interactions are the exam points.

Mechanism

Bind the 50S ribosomal subunit → inhibit protein synthesis (bacteriostatic). Erythromycin is also a motilin agonist (prokinetic, off-label).

The agents

Clarithromycinmacrolide

CAP (with amoxicillin), H. pylori; strong CYP3A4 inhibitor.

Azithromycinlong t½

OD short course

Chlamydia (2nd line), atypicals.

Erythromycinolder

Pregnancy macrolide of choice; most GI upset; neonatal pyloric stenosis.

Adverse effects

QT prolongationserious

Additive with other QT drugs.

GI upset (erythromycin worst)common

Cholestatic hepatitis; ↑pyloric stenosis risk (neonatal erythromycin)common

Cautions & contraindications

Concurrent QT-prolonging drugsall

Clarithromycin + simvastatinall

CYP3A4 → rhabdomyolysis (stop statin during course).

Interactions

CYP3A4 inhibitors (clarithromycin/erythromycin): statins, warfarin, DOACs, digoxin

Monitoring & kinetics

ECG (QT) if risk; interactions

Oral / IV.

Choosing it

Spectrum: Gram-positive (Strep, some Staph) + ATYPICALS (Mycoplasma, Chlamydia, Legionella) + some Gram-negatives (azithromycin). Used when penicillin-allergic.

Source: BNF — Macrolides