Interactions occur when one drug modifies the action of another. This interaction may either increase or decrease the second drug’s action. Sometimes these interactions result in unwanted effects, but some interactions are beneficial and can be exploited therapeutically.

Drug interaction can be described as physicochemical, relating to the properties of the drug or its pharmaceutical preparation, pharmacokinetic due to alterations in the way the body handles the drug or pharmacodynamic where the activity of one drug is affected. The chance of a significant interaction increases markedly with the number of drugs used and the effects of any interaction are often exaggerated in the presence of disease or coexisting morbidity.