[Overview of drug interactions. Combining drugs correctly!]

Drug interactions may be of two basic types: pharmacodynamic and pharmacokinetic. Pharmacodynamic interactions may be direct, in that two drugs intervene synergistically or antagonistically in the same biological system, or indirect, meaning that one drug may modify a parameter of central importance to a system, which in turn may influence the activity of the second drug. Of the two possibilities pharmacokinetic interactions are the more difficult to recognize since they have nothing to do with the biological action proper. Here, the absorption and metabolism of the drug are the most important factors. A compounding factor is the fact that the activity of a drug may show marked individual, genetically determined, differences.