title: Drug interactions--principles, examples and clinical consequences.
abstract: BACKGROUND
  Drug interactions can have desired, reduced or unwanted effects. The probability of interactions increases with the number of drugs taken. The high rate of prescribed drugs in elderly patients (65-year-old patients take an average of 5 drugs) increases the likelihood of drug interactions and thus the risk that drugs themselves can be the cause of hospitalization. According to meta-analyses, up to 7% of hospitalizations are drug-related.
  
  
  METHODS
  Selective literature review.
  
  
  RESULTS
  Drug interactions occur on pharmacodynamic and pharmacokinetic levels. Examples of pharmacodynamic interactions are simultaneous administration of a NSAID and phenprocoumon (additive interaction), or of aspirin and ibuprofen (antagonistic interaction). Pharmacokinetic interactions occur at the levels of absorption (e.g., levothyroxine and neutralizing antacids), elimination (e.g., digoxin and macrolides), and metabolism, as in the competition for cytochrome P450 enzymes (e.g., SSRIs and certain beta-blockers).
  
  
  CONCLUSIONS
  The systematic knowledge of drug interaction, in particular on the level of absorption, elimination, transport and drug metabolism may help to prevent adverse effects. Predicting pharmacodynamic interactions often demands a deeper understanding of the mechanisms of effect. Electronic prescribing systems are helpful.
authors.affiliation: Institute of Clinical and Experimental Pharmacology, University Medical Center Schleswig-Holstein, Kiel. cascorbi@pharmakologie.uni-kiel.de
authors.forename: Ingolf
authors.initials: I
authors.lastname: Cascorbi
mesh_terms: D004347:Drug Interactions; D064420:Drug-Related Side Effects and Adverse Reactions / Q000175:diagnosis* / Q000453:epidemiology / Q000517:prevention & control*; D006801:Humans; D015995:Prevalence