Mechanism of Action

  • Member of the tetracycline family of antibiotics.  Act by inhibiting bacterial protein synthesis.  Mainly useful in treating intracellular bacterial pathogens e.g. Chlamydia, Mycoplasma; but can also be useful at treating H influenzae and Strep pneumoniae
    • Used in treating pneumonia (particularly in older patients)
  • Long acting (half life 18-22 hours), oral tetracycline


  • Circulates mainly protein bound (90%)
  • Metabolised mainly by the liver and excreted in the bile/urine


  • Pregnancy, SLE, Myaesthenia gravis, porphyria

Side effects

  • Photophobia- avoid sun beds/overexposure to sun; use protection
  • Stomach upset/indigestion/abdominal pain
  • Diarrhoea
  • Headache
  • Thrush
  • Throat irritation (take with a large glass of water; sitting upright/standing)
  • NB Take doxycycline eithe 1 hour before or 2 hours after eating or taking any antacids, iron tablets, (basically medications containing metal compounds)
  • Can also cause liver toxicity; benign intracranial hypertension

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