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[Pharmacology](https://durgical.com/en/pharmacology/) · Class

# Macrolides

Large-ring antibiotics that plug the exit tunnel of the bacterial ribosome. They cover atypical pneumonia, chlamydia and many streptococci.

Updated 2026-11-01

## What they are

Antibiotics built on a large lactone ring: erythromycin, the original, and its newer relatives clarithromycin and azithromycin. They are mainly bacteriostatic and reach high levels inside cells, which makes them useful against intracellular bacteria.

## Mechanism of action

They bind the 23S rRNA of the 50S ribosomal subunit, at the entrance of the tunnel through which the new protein leaves. The growing chain is blocked, protein synthesis stops and the bacterium cannot multiply. Resistance comes mainly from methylation of the 23S rRNA target (erm genes) or from efflux pumps, and it crosses between members of the class.

Target: 23S rRNA of the bacterial 50S ribosomal subunit (exit tunnel)

## Key points

- Cover atypical bacteria (Mycoplasma, Chlamydia, Legionella) that β-lactams miss.
- Clarithromycin and erythromycin strongly inhibit CYP3A; azithromycin hardly does, so it has far fewer drug interactions.
- All can prolong the QT interval; avoid combining with other QT-prolonging drugs.
- Erythromycin also stimulates the gut motilin receptor, which explains its cramps and diarrhoea.
- On the WHO AWaRe list azithromycin, clarithromycin and erythromycin belong to the «Watch» group: higher resistance risk, use with care.

## Drugs in this class

- [**Azithromycin** A macrolide that lingers for days in the tissues, so short courses work. Used for atypical pneumonia, chlamydia… Bacterial ribosome, 50S subunit (23S rRNA, exit tunnel)](https://durgical.com/en/pharmacology/azithromycin/)
- [**Clarithromycin** A macrolide used against H. pylori, chest infections and Mycobacterium avium. It strongly blocks CYP3A, so it has many… Bacterial ribosome, 50S subunit (23S rRNA, exit tunnel)](https://durgical.com/en/pharmacology/clarithromycin/)
- [**Erythromycin** The first macrolide. Still used as an eye ointment for newborns and as an alternative to penicillin; it also stirs the… Bacterial ribosome, 50S subunit (23S rRNA, exit tunnel)](https://durgical.com/en/pharmacology/erythromycin/)

## Others in the class

- roxithromycin
- spiramycin
- fidaxomicin (related macrocycle for C. difficile)

## Test yourself

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

## Where on the ribosome do macrolides bind?

On the 23S rRNA of the 50S subunit, in the exit tunnel of the nascent protein, so the chain cannot grow.

## Which macrolide has the fewest CYP3A interactions?

Azithromycin. Clarithromycin and erythromycin are strong CYP3A inhibitors.

## Which cardiac risk do all macrolides share?

QT prolongation, with a risk of torsades de pointes.

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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 - AZITHROMYCIN tablet, film coated](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=f6a52c0b-45db-422e-8c22-e867a11b4439) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - CLARITHROMYCIN tablet, film coated](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=d836ae7e-fdbf-4dcb-a90d-ede1dcbc3e67) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - ERYTHROMYCIN LACTOBIONATE injection, powder, for solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=ee345752-0f0c-c69b-e053-2995a90a7dac) (DailyMed (U.S. National Library of Medicine), 2026); [eEML - Electronic Essential Medicines List](https://list.essentialmeds.org/?query=azithromycin) (World Health Organization, 2025).

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Representative model, for educational purposes only.

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