As the prevalence of diabetes rises, the use of antidiabetic drugs becomes more frequent. Thus, focusing on the effects of these drugs on water-sodium balance and electrolyte regulation is necessary. This review discusses the effects and the mechanisms behind them. Several sulfonylureas, such as chlorpropamide, methanesulfonamide, and tolbutamide, exhibit water-retaining properties. Other sulfonylureas, such as glipizide, glibenclamide, acetohexamide, and tolazamide, are not antidiuretic or even diuretic. Numerous clinical studies showed that metformin can reduce serum magnesium concentrations and may have an effect on the cardiovascular system, but the specific mechanism remains to be discussed. Different opinions exist about the mechanisms of thiazolidinedione-induced fluid retention. Sodium-glucose cotransporter 2 inhibitors can cause osmotic diuresis and natriuresis and elevated serum potassium and magnesium concentrations. Glucagon-like peptide-1 receptor agonists and dipeptidyl peptidase-4 inhibitors can enhance urine sodium excretion. At the same time, increased urinary sodium caused by sodium-glucose cotransporter 2 inhibitors, glucagon-like peptide-1 agonists and dipeptidyl peptidase-4 inhibitors reduce blood pressure and plasma volume, thereby protecting the heart. Insulin has a sodium-retaining effect and is also associated with hypokalemia, hypomagnesemia, and hypophosphatemia. Several of the aforementioned pathophysiological changes and mechanisms have been discussed, and conclusions have been drawn. However, further investigation and discussion are still warranted.