Drug interactions can be classified according to their pharmacodynamic and pharmacokinetic mechanisms. Pharmacodynamic interactions are observed when two drugs share a common effect or have the same effect on different receptors of a common function. They can be predicted if the elementary effects of each drug are known. Such are pharmacodynamic interactions are particularly interesting as they are selective for the common effect(s). Pharmacokinetic interactions are more difficult to predict. They occur when one drug modifies the pharmacokinetic parameters of a second drug. Modification may involve variations in oral absorption, tissue distribution, rate of metabolism and/or rate of renal excretion. This interaction cannot be selective as drug concentrations are modified, affecting all the concentration-dependent effects. Mastering drug interactions involve the knowledge of the underlying mechanisms. Answers to following questions are needed: does the association produce conjugated effects? can one drug modify pharmacokinetic parameters of another? Potentially toxic effects may be suspected when at least one drug involved is known for its toxicity. Risk is increased when it also exhibits a narrow range between active and toxic concentrations. Many databases in printed form or as interactive software provide information on drug interactions. Clinicians can also consult a Pharmacovigilance Center for a safe prescription procedure. Finally, the main danger lies in the simultaneous prescriptions by different practitioners unaware of their colleagues prescription. In this case, the pharmacist plays an important role of evaluation of the drugs prescribed. Ann Pharm Fr; 58(4):247-53, 2000 Jul.

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