Diuretics were invented 70 years ago to reduce body swelling due to water retention. They are based on relatively simple chemical formulas that work by encouraging the kidneys to expel more urine, thereby reducing the amount of water in the body. Thus, diuretics are popularly known as ‘water pills’.

Commonly prescribed diuretics are bendrofluazide, chlorothiazide, chlorthalidone, cyclopenthiazide, hydrochlorothiazide and indapamide.

That diuretics lower blood pressure was discovered by accident in the 1940s and, even today, it is not known how they work to reduce blood pressure.

It is claimed that diuretics prevent the cardiovascular illness caused by high blood pressure, such as stroke and heart failure, but the actual reduction in death rates appears to be only a modest 10 per cent (JAMA, 1997; 277: 739-45). The picture is further complicated by two recent pieces of evidence.

1. Low blood pressure as well as high blood pressure can lead to cardiovascular problems and premature death - and one cause of low blood pressure is diuretic drugs (Ann Intern Med, 2002; 136: 438-48).

2. Diuretics can have serious side-effects, such as cancer of the kidneys, which may further worsen death rates (Cardiovasc Drugs Ther, 2000; 14: 407-9).

Other side-effects of diuretics include impotence, dizziness on standing up (due to low blood pressure), blood disorders, skin reactions, gout, pancreatitis, and depletion of potassium, magnesium, coenzyme Q10 and zinc. There is also evidence that diuretics may be 'harmful' in people with coronary heart disease (J Cardiovasc Pharmacol, 1990; 16: 58-63).

Over 3 per cent of patients on diuretics find the accompanying side-effects intolerable (Pharmacotherapy, 2001; 21: 940-53).