These drugs are used to treat hypertension (high blood pressure). It is not known exactly how they work in this condition. Usually low doses are used and after a few days they do not cause an increased production of urine. It is known that they increase the effects of some other groups such as ACE inhibitors and Angiotensin 2 receptor blockers.

Diuretics are also used in the treatment of heart failure. In heart failure the body retains too much salt (sodium) and water. Fluid builds up in the lungs making them stiff and causing breathlessness. Diuretics increase both the loss of salt and water improving symptoms.