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.

About one in six inpatient drug charts contain a significant drug interaction, one-third of which are potentially serious. An uncomplicated general anaesthetic for a relatively routine case may use ten or more different agents that may interact with one another or, more commonly, with the patient's concurrent medication.

Pharmaceutical

These interactions occur because of a chemical or physical incompatibility between the preparations being used. Sodium bicarbonate and calcium will precipitate out of solution as calcium carbonate when co-administered in the same giving set. However, one agent may inactivate another without such an overt indication to the observer; insulin may be denatured if prepared in solutions of dextrose and may, therefore, lose its pharmacological effect. Drugs also may react with the giving set or syringe and therefore need special equipment for delivery, such as a glass syringe for paraldehyde administration.