Reviewed by MedNext Clinical Team
Doxycycline is a second-generation tetracycline antibiotic with a broad spectrum of activity and an impressive range of clinical indications. Its improved oral bioavailability, longer half-life (allowing once or twice daily dosing), and better tolerability compared with earlier tetracyclines have cemented its place as a versatile agent in both primary and secondary care [1].
Spectrum and Mechanism
Doxycycline inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit, blocking aminoacyl-tRNA attachment. It is active against a wide range of organisms including atypical pathogens (Chlamydia, Mycoplasma, Rickettsia, Coxiella), gram-positive cocci, many gram-negative organisms, spirochaetes (Borrelia burgdorferi), and some protozoa (Plasmodium falciparum in combination with other agents).
Common Indications
- Respiratory tract infections: Community-acquired pneumonia (especially atypical), exacerbations of COPD
- Skin and soft tissue: Acne vulgaris (long-term low-dose), rosacea, cellulitis caused by susceptible organisms
- Sexually transmitted infections: Chlamydia trachomatis (100 mg twice daily for 7 days), non-gonococcal urethritis, lymphogranuloma venereum
- Lyme disease: First-line for early Lyme disease and Lyme arthritis in adults
- Malaria prophylaxis: 100 mg daily starting 1–2 days before travel, continued for 4 weeks after return [1]
- Tick-borne illnesses: Rocky Mountain spotted fever, Q fever, brucellosis
Administration: Key Patient Counselling Points
Take Upright with Plenty of Water
Doxycycline tablets and capsules can cause oesophageal ulceration if they lodge in the oesophagus — a painful and preventable complication. Patients must take each dose with a full glass of water (at least 200 mL) while sitting or standing upright, and remain upright for at least 30 minutes afterwards. Doxycycline should not be taken immediately before lying down or at bedtime.
Dairy and Antacid Interactions
Unlike older tetracyclines, doxycycline absorption is only minimally affected by food and dairy products. It can generally be taken with food to reduce GI side effects. However, antacids containing calcium, magnesium, or aluminium, iron supplements, and zinc chelate doxycycline and significantly reduce its absorption — these should be taken at least 2–3 hours apart.
Photosensitivity
Doxycycline is one of the most photosensitising antibiotics. Patients should be warned to use high-SPF sunscreen, wear protective clothing, and avoid prolonged sun exposure during treatment. This is particularly relevant for patients on long-term doxycycline for acne or malaria prophylaxis in tropical destinations.
Contraindications and Cautions
Doxycycline is contraindicated in pregnancy (risk of foetal bone and tooth development impairment) and in children under 8 years of age (permanent tooth discolouration). It is not renally cleared and is therefore one of the safer antibiotics in renal impairment, though hepatic impairment warrants caution due to hepatic metabolism [1].
References
- Gaillard T, Madamet M, Pradines B. Tetracyclines in malaria. Malar J. 2015;14:445.