Factors that affect drug action (contd.)

FACTORS THAT AFFECT DRUG ACTION

  • Route of administration: Besides kinetics, in some cases route may alter action- eg. MgSO4 – orally purgation, skin – decrease swelling, i/v – CNS and cardiac depression  Iodine – antiseptic and antigoitre
  • Environment – Insecticides, tobacco smoke, charcoal broiled meat – induce metabolism.
    • Hypnotics taken at night at bed work well.
  • Psychological – beliefs, attitudes, expectations – more in humans
    • Placebo (Latin – I shall please) used in trials and to treat otherwise normal patients (hypochondriacs)
    • Placebos induce endorphins in brain analgesia

Disease states

  • Diseases of GI tract –orally given drugs - altered absorption
  • Liver diseases – Bioavailability of drugs with First pass metabolism
  • Increased – Renal disease affects excretion of drugs
  • In all the above – change of dose needed.
  • Immunocompetence – Normally in very young and very old patients immunity is compromised. So, bacteriostatic to be replaced by bactericidals
  • Drug interactions – physical or chemical inactivation or incompatibility
  • Food drug interactions – availability of food delays absorption.
  • Ingredients of food may cause specific interactions
  • Pharmacological Interactions – may be beneficial – epinephrine + local anaesthetics preanaesthetics, antibacterial combinations
  • penicillin + probenecid, ampicillin + sulbactam harmful interactions - aminoglycosides with Neuromuscular blocking agents

Enzyme inhibitors and inducers

  • Tolerance and Tachyphylaxis – Tolerance is the requirement of higher dose to produce given response –
  • Natural – individual less sensitive to drug (rabbits)
  • Acquired – by repeated use of the drug in an individual who was initially responding – may be due to faster elimination of the drug on long use or changes in receptor sensitivity (downregulation)
  • Cross tolerance – tolerance among closely related drugs ( morphine and pethidine)
  • Tachyphylaxis - rapid development of tolerance – after only a few doses
  • Seen in ephedrine, tyramine – which deplete stores of catecholamines
Last modified: Wednesday, 25 April 2012, 7:17 AM