The sulphonylureas and the biguanides are widely used as adjuncts to dietary measures in the treatment of non-insulin-dependent (type 2) diabetes mellitus (NIDDM). Adverse effect profiles differ markedly between the sulphonylureas and biguanides, reflecting differences in chemical structure and mode of action. Sulphonylureas are generally well tolerated, although pharmacokinetic differences between these agents have important clinical implications. The main adverse effect associated with sulphonylureas is hypoglycaemia. This effect is a predictable consequence of the principal pharmacological effect of these drugs, i.e. sensitisation of the islet beta-cell to glucose, resulting in enhanced endogenous insulin secretion. Sulphonylurea-induced suppression of hepatic glucose production may cause profound and protracted hypoglycaemia, especially in elderly patients, in individuals with intercurrent illnesses and reduced caloric intake, or when taken in combination with other compounds with hypoglycaemic potential, e.g. alcohol (ethanol). Sulphonylureas with a longer duration of action, notably chlorpropamide and glibenclamide (glyburide), are more liable to induce serious hypoglycaemia, particularly when drug elimination is reduced by renal impairment. Other drugs such as salicylates may potentiate the actions of sulphonylureas, thereby increasing the risk of hypoglycaemia. Biguanide therapy is associated with alterations in lactate homeostasis which under certain clinical circumstances may result in fatal lactic acidosis. Phenformin is associated with a markedly greater risk of lactic acidosis than metformin. Phenformin has been withdrawn in many countries for this reason. All biguanides must be avoided in patients with renal impairment, hepatic dysfunction and cardiac failure--conditions where drug accumulation or disordered lactate metabolism may predispose to lactic acidosis. Phenformin should not be given to individuals who exhibit a severe, genetically conferred hepatic defect of hydroxylation which impedes metabolism of this drug. Less seriously, the biguanides are associated with a relatively high incidence of gastrointestinal adverse effects which limit compliance. Acarbose, a competitive inhibitor of intestinal alpha-glucosidases, has recently been introduced. In contrast to the sulphonylureas and biguanides, acarbose has not been associated with life-threatening adverse effects. This reflects the low systemic absorption of the drug and, predictably, its principal unwanted effects are gastrointestinal disturbances resulting from iatrogenic carbohydrate malabsorption.