Adverse drug reactions (see below) are common in the elderly. This is frequently a consequence of multiple drug prescribing which leads to the occurrence of drug-drug interactions. Drug interactions represent a change in either the magnitude or duration of action of one drug caused by the presence of a second. This may enhance or reduce the efficacy of one or both of the drugs or a new effect may appear which is not seen with either of the drugs alone. Interactions may be pharmacokinetic or pharmacodynamic. The most important adverse interactions occur with drugs that have easily recognisable toxicity and a low therapeutic index (i.e. the dose or plasma concentration of drug which is effective lies close to that which causes toxicity) (Lin and Lu, 1998).
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