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Drug interactions occur when one drug modifies the actions of another drug in the body. Drug interactions can result from pharmacokinetic alterations (eg, changes in absorption, distribution, and elimination), pharmacodynamic changes (additive, synergistic, or antagonistic effects), or a combination of both. Interactions between drugs in vitro (eg, precipitation when mixed in solutions for intravenous administration) are usually classified as drug incompatibilities, not drug interactions.

Although hundreds of drug interactions have been documented, relatively few are of enough clinical significance to constitute a contraindication to simultaneous use or to require a change in dosage. Some of these are listed in Table 62–1. In patients taking many drugs, however, the likelihood of significant drug interactions is increased. Elderly patients have a high incidence of drug interactions because they commonly take multiple medications and they often have age-related changes in drug clearance.

TABLE 62–1Some important drug interactions.

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