💊 Tetracyclines

Tetracyclines are a class of broad-spectrum bacteriostatic antibiotics. They are effective against a wide range of Gram-positive and Gram-negative bacteria, as well as atypical organisms.

Tetracyclines inhibit bacterial protein synthesis by reversibly binding to the 30S ribosomal subunit. This prevents the attachment of aminoacyl-tRNA to the ribosomal acceptor site, thereby blocking peptide chain elongation.

They are used for various infections, including acne vulgaris, rosacea, community-acquired pneumonia (especially atypical pathogens), Lyme disease, and some sexually transmitted infections (e.g., chlamydia). Doxycycline is also used for malaria prophylaxis.

Tetracyclines are contraindicated in children under 12 years of age due to the risk of permanent tooth discolouration and enamel hypoplasia. They are also contraindicated in pregnancy and breastfeeding due to effects on fetal bone and tooth development. Known hypersensitivity is an absolute contraindication.

Common side effects include gastrointestinal disturbances (nausea, vomiting, diarrhoea), photosensitivity (severe sunburn), and oesophageal irritation/ulceration if not taken with sufficient water. Serious but rare effects include hepatotoxicity, pseudotumour cerebri (benign intracranial hypertension), and severe skin reactions.

No routine blood monitoring is typically required for short courses. For prolonged therapy, particularly in patients with pre-existing hepatic or renal impairment, liver and renal function tests may be considered. Patients should be advised to report any visual disturbances or severe headaches.

Always check for pregnancy, breastfeeding, and age (avoid under 12). Counsel patients on taking with plenty of water, sitting upright, and avoiding lying down for at least 30 minutes to prevent oesophageal irritation. Advise on strict sun protection due to photosensitivity. Warn about interactions with dairy, antacids, and iron supplements (take separately).

Doxycycline is a commonly used tetracycline. Key contraindications are pregnancy, breastfeeding, and children under 12. Photosensitivity is a major adverse effect. Absorption is impaired by divalent cations (calcium, iron, magnesium), so advise patients to avoid dairy, antacids, and iron supplements around the time of dosing. Can cause benign intracranial hypertension.

SBA questions often revolve around the contraindications in children and pregnancy, and the mechanism behind these. Photosensitivity is a classic adverse effect tested. Drug interactions, particularly with dairy products and antacids, are also common themes.