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The topic of drug interactions is strikingly complex and rapidly evolving. Even experts in this area of pharmacology must rely on print and electronic resources pertaining to drug interactions on a regular basis. With these facts in mind, this chapter focuses on a wide variety of principles, with selected drug interactions utilized to illustrate these principles, as opposed to providing extensive lists of potential drug interactions. Furthermore, there will be an attempt to provide a stratification of risk, putting the greatest emphasis on the drug interactions with the greatest risk to individual patients.


The most basic definition of a drug interaction is when two drugs are being administered simultaneously in a patient, and one drug alters the other's serum/tissue levels or mechanism of action. The drug interaction can affect the efficacy or increase the likelihood of adverse effects from one or both drugs. Furthermore, a drug interaction can occur without clinically evident alteration of efficacy or adverse effects. In contrast, drug interactions which induce either a loss of drug efficacy or new adverse effects are known as adverse drug interactions. The topic of drugs interactions is important to physicians in all aspects of medicine regardless if the clinician (a) prescribes the drug “responsible” for a given drug interaction, or (b) the drug previously prescribed by this same clinician is the “victim” of a drug prescribed by another clinician.

Print and Electronic Resources

In reviewing this chapter, the reader is encouraged to strive for a “recognition recall” level of memory; no one can possibly master and retain all important drug interactions. Through learning key principles that assist in attaining the broadest possible understanding of the multitudes of potential drug interactions, a clinician may best be able to interpret and react to new clinician situations involving potential drug interactions. One should always attempt make things “make sense” using the general principles that follow. It is always acceptable to (1) call a drug information pharmacist, (2) call or e-mail an expert in drug interactions, or (3) look up possible interactions in various print and electronic resources. Selected resources for drug interactions are listed in Table 236-1.

Table 236-1 Databases for Information on Drug Interactions


The topic of P-glycoprotein and the role in drug interactions has ...

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