Non-insulin-dependent (type 2) diabetes mellitus (NIDDM) affects middle-aged or elderly people who frequently have several other concomitant diseases, especially obesity, hypertension, dyslipidaemias, coronary insufficiency, heart failure and arthropathies. Thus, polymedication is the rule in this population, and the risk of drug interactions is important, particularly in elderly patients. The present review is restricted to the interactions of other drugs with antihyperglycaemic compounds, and will not consider the mirror image, i.e. the interactions of antihyperglycaemic agents with other drugs. Oral antihyperglycaemic agents include sulphonylureas, biguanides--essentially metformin since the withdrawn of phenformin and buformin--and alpha-glucosidase inhibitors, acarbose being the only representative on the market. These drugs can be used alone or in combination to obtain better metabolic control, sometimes with insulin. Drug interactions with antihyperglycaemic agents can be divided into pharmacokinetic and pharmacodynamic interactions. Most pharmacokinetic studies concern sulphonylureas, whose action may be enhanced by numerous other drugs, thus increasing the risk of hypoglycaemia. Such an effect may result essentially from protein binding displacement, inhibition of hepatic metabolism and reduction of renal clearance. Reduction of the hypoglycaemic activity of sulphonylureas due to pharmacokinetic interactions with other drugs appears to be much less frequent. Drug interactions leading to an increase in plasma metformin concentrations, mainly by reducing the renal excretion or the hepatic metabolism of the biguanide, should be avoided to limit the risk of hyperlactaemia. Owing to its mode of action, pharmacokinetic interferences with acarbose are limited to the gastrointestinal tract, but have not been extensively studied yet. Pharmacodynamic interactions are quite numerous and may result in a potentiation of the hypoglycaemic action or, conversely, in a deterioration of blood glucose control. Such interactions may be observed whatever the type of antidiabetic treatment. They result from the intrinsic properties of the coprescribed drug on insulin secretion and action, or on a key step of carbohydrate metabolism. Finally, a combination of 2 to 3 antihyperglycaemic agents is common for treating patients with NIDDM to benefit from the synergistic effect of compounds acting on different sites of carbohydrate metabolism. Possible pharmacokinetic interactions between alpha-glucosidase inhibitors and classical antidiabetic oral agents should be better studied in the diabetic population.