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

# Tetracyclines

Four-ring antibiotics that block tRNA docking on the 30S ribosome. Key for rickettsiae, chlamydia, leptospirosis and malaria prevention.

Updated 2026-11-01

## What they are

Broad-spectrum, mainly bacteriostatic antibiotics with four fused rings: tetracycline, doxycycline, minocycline, demeclocycline and the glycylcycline tigecycline. They act on intracellular bacteria such as Rickettsia and Chlamydia and on spirochaetes.

## Mechanism of action

They bind the 30S ribosomal subunit (16S rRNA) and stop aminoacyl-tRNA from settling in the A site, so no new amino acid is added. Bacteria resist by pumping the drug out or by ribosomal protection proteins; tigecycline was designed to escape both.

Target: 16S rRNA of the bacterial 30S ribosomal subunit (A site)

## Key points

- Bind calcium: they stain developing teeth and slow bone growth, so they are avoided in the second half of pregnancy and in children up to 8 years (doxycycline is still used in life-threatening disease such as Rocky Mountain spotted fever).
- Antacids, calcium, magnesium, iron and dairy products bind them in the gut and cut absorption.
- Photosensitivity and pill oesophagitis: plenty of water and stay upright after the dose.
- Doxycycline treats rickettsial infections (such as Rocky Mountain spotted fever) and covers Chlamydia, leptospirosis and malaria prevention.
- Demeclocycline causes nephrogenic diabetes insipidus, a side effect sometimes used in SIADH.

## Drugs in this class

- [**Doxycycline** The most used tetracycline: for rickettsial fevers, chlamydia, leptospirosis, cholera and malaria prevention. Taken… Bacterial ribosome, 30S subunit (16S rRNA, A site)](https://durgical.com/en/pharmacology/doxycycline/)
- [**Minocycline** A fat-soluble tetracycline used for acne and some resistant infections. Known for dizziness, skin pigmentation and… Bacterial ribosome, 30S subunit (16S rRNA, A site)](https://durgical.com/en/pharmacology/minocycline/)
- [**Tetracycline** The parent drug of its class. On the WHO list as an eye ointment for conjunctivitis; by mouth it is used for acne and… Bacterial ribosome, 30S subunit (16S rRNA, A site)](https://durgical.com/en/pharmacology/tetracycline/)
- [**Tigecycline** An IV glycylcycline that escapes the usual tetracycline resistance. Kept for complicated infections because of a boxed… Bacterial ribosome, 30S subunit (16S rRNA, A site)](https://durgical.com/en/pharmacology/tigecycline/)
- [**Demeclocycline** A tetracycline better known for a side effect: it makes the kidney resist ADH, which is why it is sometimes used in… Bacterial ribosome, 30S subunit (16S rRNA, A site)](https://durgical.com/en/pharmacology/demeclocycline/)

## Others in the class

- omadacycline
- eravacycline
- oxytetracycline
- chlortetracycline

## Test yourself

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

## Which ribosomal step do tetracyclines block?

The binding of aminoacyl-tRNA to the A site of the 30S subunit.

## Why are tetracyclines avoided in young children and late pregnancy?

They bind calcium in growing teeth and bone, causing permanent tooth discoloration and slowed bone growth.

## Why should doxycycline not be swallowed with milk or an antacid?

Calcium, magnesium, aluminium and iron chelate it in the gut and reduce its absorption.

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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 - DOXYCYCLINE HYCLATE capsule](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=fbc1924f-d647-465a-b8b2-25de332c5896) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - TETRACYCLINE HYDROCHLORIDE capsule](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=00acf413-c5d2-4a54-9d0d-56266cbc9edc) (DailyMed (U.S. National Library of Medicine), 2026); [DailyMed - TIGECYCLINE injection, powder, lyophilized, for solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=50605463-2dc2-4b27-bc51-4d87bb9a8539) (DailyMed (U.S. National Library of Medicine), 2026); [eEML - Electronic Essential Medicines List](https://list.essentialmeds.org/?query=doxycycline) (World Health Organization, 2025).

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Representative model, for educational purposes only.

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