The frequency of adverse drug interactions increases disproportionately with the increase in the number of drugs given to patients. It was shown that 40% of patients given 16 drugs experienced an adverse drug interaction, compared with 5% of patients given fewer than 6 drugs. The magnitude of the drug interaction problem increases substantially in anaesthetised patients because of: (i) the increased use of multiple drugs in the preoperative and intraoperative periods; and (ii) the growing population of geriatric patients who, in addition to having diminished drug metabolising capacity, are often prescribed multiple medications for concomitant medical illness.Drug interactions with volatile and intravenous anaesthetics can be divided into those that are pharmacokinetic and pharmacodynamic in nature. Pharmacokinetic interactions occur when the absorption, distribution, metabolism or excretion of a drug is altered by the coadministration of a second drug. Pharmacodynamic interactions involve a change in the pharmacological effect of a drug as a result of the action of a second drug at receptor sites.An interaction between drugs, if not recognised and corrected, can result in decreased drug efficacy or serious toxicity. It is essential that careful scrutiny of the patient's drug history is an integral part of the preoperative assessment.