[Drug interactions and clinical relevance: it all began with cheese]

In a drug interaction, the effects of one drug can be increased or decreased or a quite new effect produced by the previous, concurrent or subsequent administration of another substance, including prescription and nonprescription drugs, food, tobacco or alcohol. The effect of the interaction can be desirable, inconsequential, or adverse. The increasing number of drugs available and the increasing use of multidrug therapeutic regimens enhance the potential for drug interactions. However, in clinical practice, most interactions are not significant or rarely of significance. It is when the interaction leads to adverse consequences that it comes to the attention of the patient and physician. Interactions may occur by pharmacokinetic or pharmacodynamics mechanisms. Pharmacokinetic interactions represent the modification of one substance (the interacting substance) on the ADME processes of absorption, distribution, metabolism or excretion of a drug (the index drug). Subsequently, it may lead to changes in the concentration of the index drug at the receptor sites. Drug interactions with a pharmacodynamic basis involve actions on the same receptor or physiological systems through either synergism or antagonism. Many drug interactions can be predicted if the pharmacodynamics effects, pharmacokinetic properties and mechanisms of action of the interacting agents are known. The most obvious interactions are those producing altered pharmacokinetic of drugs with a low therapeutic index (oral anticoagulants, antidiabetic drugs, digoxin, benzodiazepines and immunosuppressant and cytotoxic drugs).