3rd August 2023

Tetracyclines

Mechanism of action

Bacteriostatic drugs inhibit bacterial growth by binding to the 30s subunit of the bacterial ribosome, preventing the translation of RNA into proteins. They are not bactericidal – like beta-lactams – and do not directly kill bacteria.

Spectrum of activity

A broad spectrum antibiotic, which covers both Gram positive and negative bacteria, can also be used to treat intracellular organisms. Due to increasing drug resistance, they are less popular than in previous years. They are still indicated in treating chlamydia, rickettsia brucella and Lyme disease. They are also used in genital infections, acne, and respiratory tract infections, especially exacerbations of chronic bronchitis, due to their activity against Haemophilus influenzae, a commonly associated pathogen.

Examples include doxycycline, minocycline, and tigecycline. They are all very similar, with doxycycline generally the macrolide of choice.

Important side effects, interactions and monitoring

Tetracyclines do not require routine monitoring. They should be used cautiously in those with hepatic and renal impairment but without precise contraindications.

A key side effect is photosensitivity; tetracycline use increases the risk of sunburn under exposure to UV light.

Tetracyclines are contraindicated in pregnant and breastfeeding women. This is because of the well-documented phenomenon of permanent discolouration of the teeth of foetuses and babies who are exposed. They are also contraindicated in children under the age of 8 for the same reason. This is known as “tetracycline teeth” – an example image below.

Tetracyclines are also associated with a risk of worsening muscle weakness in myasthenia gravis and exacerbating the symptoms of systemic lupus erythematosus. So they should be used with caution in these patients.

Examples of tooth staining – “tetracycline teeth”.

Image Source: Wikimedia Commons (2019). Examples of tooth staining, Licensed under a creative commons licence. Available for free at: https://commons.wikimedia.org/wiki/File:Examples_of_tooth_staining.jpg (Accessed: 25 September 2022).