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)
Drugs in this class
- DoxycyclineThe most used tetracycline: for rickettsial fevers, chlamydia, leptospirosis, cholera and malaria prevention. Taken…Bacterial ribosome, 30S subunit (16S rRNA, A site)
- MinocyclineA fat-soluble tetracycline used for acne and some resistant infections. Known for dizziness, skin pigmentation and…Bacterial ribosome, 30S subunit (16S rRNA, A site)
- TetracyclineThe 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)
- TigecyclineAn IV glycylcycline that escapes the usual tetracycline resistance. Kept for complicated infections because of a boxed…Bacterial ribosome, 30S subunit (16S rRNA, A site)
- DemeclocyclineA 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)
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.
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 (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - TETRACYCLINE HYDROCHLORIDE capsule (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - TIGECYCLINE injection, powder, lyophilized, for solution (DailyMed (U.S. National Library of Medicine), 2026); eEML - Electronic Essential Medicines List (World Health Organization, 2025).
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Representative model, for educational purposes only.