Cross tolerance- It is the development of tole rance to pharmacologically related drugs, e.g. alcoholics are relatively tolerant to barbiturates and general anaesthetics. Closer the two drugs are, more complete is the cross tolerance between them, e.g.-
There is partial cross tolerance between morphine and barbiturates but complete cross tolerance between morphine and pethidine.
Mechanisms responsible for development of tolerance are incompletely understood. However, tolerance may be:
(i) Pharmacokinetic/drug disposition tole rance-the effective concentration of the drug at the site of action is decreased, mostly due to enhancement of drug elimination on chronic use, e.g. barbiturates, carbama zepine, amphetamine.
(ii) Pharmacodynamic/cellular tolerance drug action is lessened; cells of the target organ become less responsive, e.g. morp hine, barbiturates, nitrates. This may be due to down regulation of receptors or weakening of response effectuation.
Tachyphylaxis (Tachy-fast, phylaxis-protectionJ 10 rapid development of tolerance when doses of c. drug repeated in quick succession result ir marked reduction in response. This is usuall;: seen with indirectly acting drugs, such as
ephedrine, tyramine, nicotine. These drugs act b' releasing catecholamines in the body, synthesis of which is unable to match the rate of release stores get depleted. Other mechanisms like slm,· dissociation of the drug from its receptor, desensi tization/internalization or down regulation or receptor, etc. and/or compensator: homeostatic adaptation.
Drug resistance Itreferstotoleranceofmicro organisms to inhibitory action of antimicrobials. e.g. Staphylococci to penicillin
There is partial cross tolerance between morphine and barbiturates but complete cross tolerance between morphine and pethidine.
Mechanisms responsible for development of tolerance are incompletely understood. However, tolerance may be:
(i) Pharmacokinetic/drug disposition tole rance-the effective concentration of the drug at the site of action is decreased, mostly due to enhancement of drug elimination on chronic use, e.g. barbiturates, carbama zepine, amphetamine.
(ii) Pharmacodynamic/cellular tolerance drug action is lessened; cells of the target organ become less responsive, e.g. morp hine, barbiturates, nitrates. This may be due to down regulation of receptors or weakening of response effectuation.
Tachyphylaxis (Tachy-fast, phylaxis-protectionJ 10 rapid development of tolerance when doses of c. drug repeated in quick succession result ir marked reduction in response. This is usuall;: seen with indirectly acting drugs, such as
ephedrine, tyramine, nicotine. These drugs act b' releasing catecholamines in the body, synthesis of which is unable to match the rate of release stores get depleted. Other mechanisms like slm,· dissociation of the drug from its receptor, desensi tization/internalization or down regulation or receptor, etc. and/or compensator: homeostatic adaptation.
Drug resistance Itreferstotoleranceofmicro organisms to inhibitory action of antimicrobials. e.g. Staphylococci to penicillin
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