doxycycline
Tetracycline antibiotic· Brands: Vibramycin, Doxylin, Periostat, Oracea, DoxyhexalIVPO
Mechanism of action
Doxycycline inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit, preventing the attachment of aminoacyl-tRNA to the mRNA-ribosome complex. It is a bacteriostatic agent with activity against a wide range of gram-positive, gram-negative, atypical, and intracellular pathogens. It also has anti-inflammatory and immunomodulatory effects, which are exploited in some non-infective indications such as rosacea and periodontitis.
Pharmacokinetics
Doxycycline is almost completely absorbed from the gastrointestinal tract, with bioavailability of about 90% when taken orally. Absorption is reduced by food, particularly dairy products, and by antacids containing calcium, magnesium, or aluminium, and iron preparations. It is widely distributed into tissues and fluids, including pleural, peritoneal, and synovial fluids, and it crosses the placenta and enters breast milk. Plasma protein binding is about 90%. It is metabolised to a small extent, and the majority is excreted unchanged in the faeces via biliary secretion, with about 20% excreted in urine. The elimination half-life is approximately 18 to 22 hours, allowing once-daily dosing. It may accumulate in renal impairment, but dose adjustment is not usually required.
Indications & dosing
Acne vulgaris
Take with water; avoid recumbency for 30 minutes.
- adult · PO: 50–100 mg OD · Take with water; avoid recumbency for 30 minutes.
Chlamydia trachomatis infection
Treat sexual partners; test of cure recommended.
- adult · PO: 100 mg BID · Treat sexual partners; test of cure recommended.
Rickettsial infections (e.g. Rocky Mountain spotted fever)
Continue for at least 2 days after fever subsides.
- adult · PO: 100 mg BID · Continue for at least 2 days after fever subsides.
Malaria prophylaxis
Start 1-2 days before travel and continue for 4 weeks after leaving endemic area.
- adult · PO: 100 mg OD · Start 1-2 days before travel and continue for 4 weeks after leaving endemic area.
Early Lyme disease
Duration usually 10-21 days; refer to local guidelines.
- adult · PO: 100 mg BID · Duration usually 10-21 days; refer to local guidelines.
Rosacea
Sub-antimicrobial dose for anti-inflammatory effect.
- adult · PO: 40 mg OD · Sub-antimicrobial dose for anti-inflammatory effect.
Contraindications & precautions
- Hypersensitivity to doxycycline or other tetracyclines · Cross-reactivity among tetracyclines may occur.
- Pregnancy · Risk of permanent tooth discolouration and impaired bone growth in the fetus; avoid unless the benefit outweighs the risk (e.g. life-threatening infection).
- Children under 8 years · Traditionally avoided due to tooth discolouration; may be considered for serious infections such as rickettsial disease, where benefit outweighs risk.
- Take with a full glass of water and remain upright for at least 30 minutes to reduce the risk of oesophageal irritation and ulceration.
- Avoid concurrent administration of antacids, iron, calcium, magnesium, zinc, or dairy products; separate by at least 2 hours to prevent chelation and reduced absorption.
- Avoid excessive sun exposure and use protective clothing and sunscreen, as photosensitivity may cause severe sunburn.
- Use with caution in hepatic impairment; monitor liver function if prolonged therapy is required.
- May reduce the efficacy of combined oral contraceptives; use alternate or additional contraception during and for 7 days after treatment.
Adverse effects
- Nausea, vomiting and diarrhoea
- Photosensitivity
- Oesophageal ulcerationserious
- Permanent tooth discolouration (in children under 8 years)serious
- Benign intracranial hypertension (pseudotumour cerebri)serious
- Stevens-Johnson syndrome / toxic epidermal necrolysisserious
Monitoring, pregnancy & toxicity
- Therapeutic response · Throughout treatment
- Gastrointestinal symptoms (nausea, vomiting, oesophagitis) · Throughout treatment
- Skin for photosensitivity · Throughout treatment
- Liver function (if long-term use) · Periodic monitoring
- Pregnancy: Contraindicated in pregnancy, particularly during the second and third trimester, due to risk of fetal toxicity, including teeth discoloration and bone growth inhibition.
- Toxicity: Gastrointestinal symptoms, such as nausea, vomiting, and diarrhoea · Supportive care, maintain hydration, and monitor electrolytes.
- Toxicity: Oesophageal irritation or ulceration · Stop the drug, administer antacids, and ensure the patient is upright when taking the medication.
- Toxicity: Photosensitivity · Advise sun avoidance and use of sunscreen.
Clinical notes
Doxycycline is indicated for infections caused by susceptible organisms, including respiratory tract infections, urinary tract infections, chlamydia, rickettsia, mycoplasma, and malaria prophylaxis. It is also used for acne vulgaris, rosacea, and as a first-line agent for suspected rickettsial infections and Q fever. It may be used for the treatment of Lyme disease. Common adverse effects include nausea, vomiting, diarrhoea, and photosensitivity, which can be severe; patients should be advised to use sun protection. Doxycycline should not be used in children under 12 years or in pregnant or breastfeeding women due to effects on tooth development and bone growth. It may interact with oral contraceptives, although the evidence is limited, and with warfarin, increasing INR. It is important to advise patients to take doxycycline with a full glass of water and to remain upright for at least 30 minutes after administration to reduce the risk of oesophageal irritation. Compliance with local guidelines and antimicrobial stewardship principles is essential.
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BNF; local formulary