Drug–drug interactions

A drug interaction occurs when the effects of a drug are altered by the effects of another drug, a vaccine, herb, foodstuff, or device. In drug–drug interactions, a precipitant drug increases or reduces the effects of an object drug by pharmaceutical, pharmacokinetic, or pharmacodynamic mechanisms. Pharmaceutical interactions occur during intravenous drug infusion; they are avoidable by infusing drugs separately. Pharmacokinetic interactions can arise from altered absorption, protein binding, cellular distribution, metabolism, or excretion of an object drug. The last two mechanisms are the most important. Pharmacodynamic interactions can be direct (antagonism or synergism at the same site of action, or summation or synergism of similar effects at different sites) or indirect (when an outcome of an action of a precipitant drug alters the effects of an object drug). Some drug–drug interactions are beneficial, through combining drugs with different beneficial mechanisms of action or using drugs to reverse or prevent adverse reactions.