Doxycycline

• Brand names: Vibramycin, Doxylar
• Route: Oral (intravenous available)

Doxycycline binds reversibly to the 30S ribosomal subunit and blocks the attachment of aminoacyl-tRNA to the acceptor site, halting protein synthesis.

It is bacteriostatic, so it is used for skin and soft-tissue infection rather than bacteraemia or endocarditis.

It is highly lipid-soluble and reaches good concentrations in skin, sinuses and prostate.

• Adults: 100 mg twice daily by mouth for 7–14 days for MRSA skin and soft-tissue infection.
• Take with a full glass of water while sitting upright to avoid oesophageal irritation; it can be taken with food, though dairy, antacids and iron reduce absorption.
• No dose adjustment is required in kidney impairment, which distinguishes it from co-trimoxazole.
• Avoided in pregnancy and generally in children under eight, though short courses are now considered acceptable in some situations.

Doses listed are typical adult regimens quoted for reference. The actual dose, route and duration are decided by a clinician for the individual patient.

The tetracycline family began with chlortetracycline, isolated by Benjamin Duggar from a Missouri soil Streptomyces in 1945.

Doxycycline was semi-synthesised at Pfizer in the 1960s to improve absorption and lengthen the half-life, and was approved in 1967.

Its role against MRSA grew in the 2000s as community-associated strains, usually susceptible to tetracyclines, spread outside hospitals.

• Photosensitivity — sunburn on limited sun exposure is common and patients should use high-factor sunscreen.
• Oesophageal ulceration if taken lying down or with too little water; nausea and abdominal discomfort.
• Tooth discolouration and effects on developing bone in young children and during pregnancy.
• Rare: benign intracranial hypertension, and Clostridioides difficile infection.

• Acne and rosacea, chlamydia and pelvic inflammatory disease, and non-gonococcal urethritis.
• Malaria prophylaxis, Lyme disease, rickettsial infections and Q fever.
• Chronic obstructive pulmonary disease exacerbations and community-acquired pneumonia.