Sulphonamide derivatives which lower the blood sugar when they are given by mouth, by enhancing the production of insulin. They are effective in treating diabetes mellitus only when some residual pancreatic beta-cell function is present. All may lead to hypoglycaemia if given in overdose and this is particularly common when long acting sulphonylureas are given to elderly patients. There is no evidence for any difference in the effectiveness of the various sulphonylureas. Tolbutamide was the first of the sulphonlyurea drugs; it has a short duration of action and is usually given twice daily. Chlorpropamide has a more prolonged action and only needs to be given once daily, but its prolonged action causes more side-effects including sensitivity reactions. Other oral hypoglycaemic agents of this family include glibenclamide, which has a duration of action intermediate between tolbutamide and chlorpropamide and also produces side-effects (in about 30 per cent of outpatients, according to a recent study). Other sulphonlyureas include acetohexamide, glibornuride, gliclazide, glipizide, gliquidone and tolazamide. Glymidine is a related compound with a similar action to the sulphonylureas. It is particularly useful in patients who are hypersensitive to sulphonylureas.