Pharm 101: Doxycycline


Tetracycline antibiotic

  • Long-acting tetracycline antibiotic
  • Broad spectrum
  • Bacteriostatic
  • Inhibits protein synthesis by binding reversibly to the 30S ribosomal subunit
  • PO or IV administration
  • Well absorbed orally
    • Absorption is mainly in upper small intestine and is impaired by multivalent cations (Ca2+, Fe2+), dairy products and antacids
  • 40-80% protein bound
  • Well distributed to body tissues, except for CSF where concentrations are 10-25% of serum
  • Cross placenta and are excreted in breast milk
  • Serum half life 16-18 hours
  • Non-renal elimination:
    • Concentration in bile is 10x serum
    • Enterohepatic circulation may contribute to maintenance of serum levels
    • No dose adjustment required in renal failure
Antimicrobial activity
  • Effective against various gram positive and gram negative bacteria
    • Ricketssiae
    • Chlamydiae
  • Mycoplasma pneumoniae
  • Combination therapy to treat H Pylori
  • Clinical uses:
    • Acne
    • Community-acquired pneumonia
    • Exacerbation of COPD
    • Malaria treatment/prophylaxis (Plasmodium falciparum)
Adverse effects
  • Gastrointestinal upset
    • Due to direct local irritation
    • Rarely, can cause oesophageal ulceration
    • Alteration in intestinal flora can result in intestinal functional disturbances, pruritus and candidiasis
  • Readily binds to calcium in newly formed bone or teeth in young children (and fetus), therefore can cause:
    • Enamel dysplasia and teeth discolouration
    • Growth inhibition or deformity of bone
  • Deranged LFTs
    • Especially in pregnancy or high doses IV
  • Photosensitivity
  • Pregnancy and young children
  • Hepatic failure
  • Drug interactions:
    • Carbamazepine, phenytoin, barbiturates, chronic alcohol ingestion shorten half-life by 50% due to hepatic enzyme induction

Pharmacology 101

Top 200 drugs

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

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