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One of the factors that can alter the response to drugs is the concurrent administration of other drugs. There are several mechanisms by which drugs may interact, but most can be categorized as pharmacokinetic (absorption, distribution, metabolism, excretion), pharmacodynamic (additive, synergistic, or antagonistic effects), or combined interactions. The general principles of pharmacokinetics are discussed in Chapters 3 and 4; the general principles of pharmacodynamics in Chapter 2.


Botanical medications ("herbals") may interact with each other or with conventional drugs. Unfortunately, botanicals are much less well studied than other drugs, so information about their interactions is scanty. Pharmacodynamic interactions are described in Chapter 64. Pharmacokinetic interactions that have been documented (eg, St. John's wort) are listed in Table 66–1.

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Table 66–1 Important Drug Interactions.

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