Pharm 101: Doxycycline

Class

Tetracycline antibiotic

Pharmacodynamics
  • Long-acting tetracycline antibiotic
  • Broad spectrum
  • Bacteriostatic
  • Inhibits protein synthesis by binding reversibly to the 30S ribosomal subunit
Pharmacokinetics
  • 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
Precautions/contraindications
  • Pregnancy and young children
  • Hepatic failure
  • Drug interactions:
    • Carbamazepine, phenytoin, barbiturates, chronic alcohol ingestion shorten half-life by 50% due to hepatic enzyme induction
References
Pharm 101 700

Pharmacology 101

Top 200 drugs

MBBS CCPU (RCE, Biliary, DVT, E-FAST, AAA) Rob is an Emergency Medicine Advanced Trainee based in Melbourne, Australia. He has special interests in medical education, ECG interpretation, and the use of diagnostic and procedural ultrasound in the undifferentiated and unwell patient.

Follow him on twitter: @rob_buttner | ECG Library |

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