Factors that affect drug action (contd.)
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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
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Environment – Insecticides, tobacco smoke, charcoal broiled meat – induce metabolism.
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Psychological – beliefs, attitudes, expectations – more in humans
Disease states
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Diseases of GI tract –orally given drugs - altered absorption
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Liver diseases – Bioavailability of drugs with First pass metabolism
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Increased – Renal disease affects excretion of drugs
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In all the above – change of dose needed.
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Immunocompetence – Normally in very young and very old patients immunity is compromised. So, bacteriostatic to be replaced by bactericidals
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Ingredients of food may cause specific interactions
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Pharmacological Interactions – may be beneficial – epinephrine + local anaesthetics preanaesthetics, antibacterial combinations
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penicillin + probenecid, ampicillin + sulbactam harmful interactions - aminoglycosides with Neuromuscular blocking agents
Enzyme inhibitors and inducers
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Tolerance and Tachyphylaxis – Tolerance is the requirement of higher dose to produce given response –
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Natural – individual less sensitive to drug (rabbits)
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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)
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Cross tolerance – tolerance among closely related drugs ( morphine and pethidine)
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Tachyphylaxis - rapid development of tolerance – after only a few doses
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Seen in ephedrine, tyramine – which deplete stores of catecholamines
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Last modified: Wednesday, 25 April 2012, 7:17 AM